Gas sensing using magnetic tunnel junction elements

ABSTRACT

Gas sensing using MTJ elements to capture/store gas concentration level data for readout at room temperature. In one embodiment, during reset the MTJ elements are heated above blocking temperatures of their storage layers while applying a first magnetic biasing force to set initial magnetic orientations. During gas sensing, reaction heat from a gas sensing element combines with control heat to raise each MTJ element&#39;s temperature from a work point temperature above its blocking temperature only when the target gas exceeds an associated concentration level, whereby a second magnetic biasing force causes the magnetic orientation to switch directions. During readout, read currents are measured to determine the MTJ elements&#39; final resistance states, which indicate their switched/non-switched states, and the resistance states are correlated with stored data to determine the measured gas concentration level. The MTJ elements are cooled after reset and gas sensing to facilitate accurate CDS readout data.

FIELD OF THE INVENTION

This invention generally relates to sensors capable of detecting atarget gas in an environment, and more particularly to gas sensingmethods implemented by semiconductor gas sensors capable of determiningconcentration levels of a target gas.

BACKGROUND OF THE INVENTION

A gas sensor (aka, gas detector) is a device configured to detect thepresence or absence of one or more target gases in a gaseous environment(e.g., a volume of air). For example, gas sensors are used to detectdangerous (e.g., flammable or toxic) gases in amounts that exceedminimum safety levels, or to detect oxygen depletion (i.e., when ambientoxygen levels fall below a predetermined concentration level). Gassensors typically interface with a safety control system that performs asafety function in response to a positive detection signal generated bya gas sensor (e.g., to automatically shut down a process, or to sound asafety alarm, when the amount of target gas exceeds or falls below apredetermined concentration level).

Gas sensors capable of quantitatively measuring the concentration levelof a target gas generally include remote-type gas sensors andcontact-type gas sensors. Remote-type IR sensors, which include infrared(IR) point sensors and IR imaging sensors, are capable of detecting atarget gas in a specified environment (i.e., the gas-filled volumecontaining the gas) without being in physical contact with the targetenvironment, and are typically used to detect or measure gas leaks inlarge area environments such as oil refineries. However, remote-type gassensors are typically expensive to produce and operate, and are thusimpractical for detecting target gasses in smaller enclosed areas. Incontrast to remote-type gas sensors, contact-type gas sensors are placedin direct contact with a monitored environment, and utilize gas sensingelements that react in a measurable way when a target gas is present inthe environment. Contact-type gas sensors are typically smaller and lessexpensive than remote-type gas sensors, and are utilized mainly inenclosed areas such as buildings or processing chambers.

Semiconductor gas sensors are contact-type sensors including gas sensingelements whose electrical resistance changes in response to a reactioncaused by the presence of a target gas, whereby detection or measurementof the target gas is achievable by way of monitoring changes in acurrent passed through the gas sensing element. The resistance change ofgas sensing elements in semiconductor gas sensors is typically caused byone of three different reaction types: (i) a chemical reaction caused bychanges in the composition or chemical structure of the gas sensingelement in response to adsorption of the target gas into the gas sensingelement; (ii) a temperature change of the gas sensing element as aresult of endothermic or exothermic (e.g., combustion-type) reaction ofthe gas sensing element with the target gas; and (iii) a temperaturechange of the gas sensing element caused by a different thermalconductivity of the target gas versus ambient gases (this effect isdependent on gas flow over the sensing element). In bothchemical-reaction-type gas sensors (i.e., semiconductor gas sensorsconfigured in accordance with reaction type (i)) and inthermal-reaction-type gas sensors (i.e., semiconductor gas sensorsconfigured in accordance with reaction types (ii) and (iii)), theresulting change in electrical resistance across the gas sensing elementis measurable by way of passing a current through the gas sensingelement, and monitoring the current for changes that are characteristicof reactions with the target gas.

Semiconductor gas sensors have an advantage over other gas sensor typesis that they can typically be produced using low-cost photolithographicfabrication processes developed for integrated circuit (IC) fabrication,and are therefore smaller and less expensive than other gas sensortypes. In many cases, semiconductor gas sensors utilize bulk Si or SOIwafers as starting materials, and include a thermally isolated membraneformed by removing silicon from the rear of the wafer by plasma or wetetching. The etch stops at the BOX of the SOI or at silicon nitridelayer formed at the surface of bulk silicon. The gas sensing element,sensors and a resistive heater are typically formed on the membrane, andcontrol circuitry of the gas sensor is typically fabricated on theadjacent bulk Si or SOI using known semiconductor processing techniques.Typical semiconductor gas sensor designs include close-membrane sensorsand membranes suspended by holding arms, with the gas sensing elementtypically disposed in a center of the membrane to improve thermalisolation.

A problem with conventional chemical-reaction-type semiconductor gassensors is that they require gas sensing elements that are limited todetecting one or a relatively small number of target gases. That is, thedetection mechanism of chemical-reaction-type semiconductor gas sensorsrequires adsorption of the target gas into the gas sensing elementmaterial, and there is no gas sensing element material that is receptiveto all gas types. Therefore, chemical-reaction-type semiconductor gassensors are either limited to one target gas (or a very small number oftarget gasses), or must include multiple gas sensor units, each unithaving a different gas sensing element materials, in order to detectmore than one target gas type.

Two chemical-reaction-type semiconductor gas sensors have been proposedin which the gas sensing element is incorporated into a magnetic tunneljunction (MTJ) element. MTJ elements typically include two ferromagneticelectrodes separated by a thin insulating layer and configured such thata resistance across the MTJ element depends on the relative orientationsof the easy axes of magnetization (herein “magnetic orientations” or“magnetic directions”) of the two ferromagnetic electrodes. The magneticorientation of one of the ferroelectric electrodes is typically fixed(e.g., using an adjacent antiferromagnetic layer) and acts as areference layer of the MTJ element, while the second ferromagneticelectrode forms a “free” layer of the MTJ element whose magneticorientation can be switched by an external magnetic field betweenparallel and anti-parallel magnetic orientations relative to thereference layer. When the magnetic orientations of the reference andfree layers are parallel, a current conductance through the tunneldielectric is relatively high (i.e., the MTJ element is in a lowresistance state), and when the magnetization vectors are anti-parallel,the current conductance is low (i.e., the MTJ element is in a highresistance state). A first MTJ-type chemical-reaction-type semiconductorgas sensor was disclosed in U.S. Pat. No. 8,826,726 (University ofCalifornia, 2014), where the free layer of a modified MTJ element wasformed with a gas adsorbing magnetic material (cobalt) that was found tocause the magnetic orientation of the free layer to flip fromanti-parallel (out-of-plane) to parallel (in-plane) when sufficientamount of a gas to be detected (hydrogen) gas was adsorbed, and wherethe modified MTJ element could be reset by heating the MTJ element todesorb the gas (i.e., heat is not used during the gas sensing phase).U.S. Pat. No. 9,097,677 (Univ. of Florida, 2015) discloses a secondMTJ-based chemical-reaction-type semiconductor gas sensor in which twoferromagnetic structures are separated by a gas-sensitive metallicinterlayer (e.g., palladium) such that a magnetic exchange couplingbetween the two ferromagnetic structures is affected by the amount ofhydrogen gas adsorbed into the metallic interlayer. Similar to otherchemical-reaction-type semiconductor gas sensors, MTJ-basedchemical-reaction-type semiconductor gas sensors are limited in thatthey can only detect a limited number of gas types. Moreover,quantitative gas measurement using conventional MTJ-basedchemical-reaction-type semiconductor gas sensor relies either ondetermining the time required for a single MTJ element to flip magneticorientations, or determining magnetization vector angle changes when thetemperature is switched from T1 to T2 value, with neither approachproviding practical and sufficiently accurate quantitative measurementresult data. Further, both U.S. Pat. Nos. 8,826,726 and 9,097,677 failto specify cooling the MTJ elements to room temperature before measuringresistance in order to determine changes to the MTJ element's magneticorientation.

Although not limited to a small number of target gasses likechemical-reaction-type semiconductor gas sensors, conventionalthermal-reaction-type semiconductor gas sensors are insufficientlyaccurate in that they require measuring resistance changes at hightemperatures. For example, combustion-type semiconductor gas sensorsutilize catalyst-type gas sensing elements (e.g., Platinum or Palladium)that are heated to the threshold temperature of a combustible targetgas, whereby the target gas exothermically reacts (oxidizes) to furtherincrease the temperature of the catalyst-type gas sensing element to areaction temperature, which is typically in the range of 100-350° C.Different target gasses have different threshold temperatures, so thecatalyst-type gas sensing element is heated to different thresholdtemperatures at different times to detect different target gasses.Sensing of the reaction temperature increase is performed either bymeasuring current changes through the gas sensing element materialsthemselves (e.g., measuring resistance of a platinum catalyst), or bymeasuring the resistance of a sensor structure formed on the membraneadjacent to the gas sensing element (e.g., a specially formed MOSFET astaught, e.g., in D. Briand et al, IEEE EDL-22(I), pp. 11-13, 2001). Ofthese options, the special MOSFET approach currently provides thehighest sensitivity (i.e., about 2%/° K at room temperature), with mostother resistive sensor approaches exhibiting less than 0.5%/° K (IEEETrans. ED vol. 52, 2005 “Temperature Sensitivity of SOI-CMOS Transistorsfor use in Uncooled Thermal Sensing”, Eran Socher at al). However, ineither case, the resistance measurement must take place during theexothermic reaction, which means that the measuring current must bepassed through a measuring element (e.g., the special MOSFET or gassensing element) that is close to the reaction temperature.Unfortunately, operation of the special MOSFET sensing transistor athigh temperatures (above 150° C.) is limited due to leakage currents(even for partially or fully depleted SOI MOS transistors, whereleakages are significantly lower than for the bulk devices). Moreover,the thermal sensitivity of the gas sensor's control circuit is stronglydecreased at elevated temperatures (e.g., the thermal sensitivity of0.35 μm transistors was observed as decreasing by approximately 50% at100° C. as compared to room temperature).

What is needed is a method for operating a thermal-reaction-typesemiconductor gas sensor that avoids the high-temperature resistancemeasurement problems associated with conventional thermal-reaction-typesemiconductor gas sensors. What is also needed method for quickly andaccurately determining the concentration level of a target (sensed) gasin a gaseous environment.

SUMMARY OF THE INVENTION

The present invention is directed to improved gas sensing methods thatutilize one or more magnetic tunnel junction (MTJ) elements and one ormore associated gas sensing elements in accordance with various noveloperating strategies that avoid the high-temperature resistancemeasurement problems associated with conventional thermal-reaction-typesemiconductor gas sensors, and/or facilitate substantially faster andmore accurate concentration level measurements of one or more targetgases in a gaseous environment using gas sensing elements of either thethermal-reaction type or the chemical-reaction type. Each of the noveloperating strategies, which are set forth in exemplary embodimentsdescribed below, implements a core methodology in which each MTJ elementis operably coupled to a gas sensing element and controlled during a gassensing phase such that a resistance state of the MTJ element switchesfrom a known initial (e.g., low) resistance value to an opposite (e.g.,high) resistance value when the gas sensing element is exposed to targetgas above a predetermined minimum concentration level, whereby each MTJelement is effectively utilized as a non-volatile memory cell thatcaptures one bit of target gas concentration level data (i.e., eitherabove or below the predetermined minimum concentration level) such thatthe data can be read out during a subsequent readout phase (i.e., by wayof determining whether or not the resistance state of the MTJ elementswitched during the gas sensing phase). As set forth below withreference to exemplary embodiments, this core methodology is modified inaccordance with various novel approaches to provide gas sensing methodsthat avoid the high-temperature resistance measurement problemsassociated with conventional thermal-reaction-type semiconductor gassensors, and/or facilitate substantially faster and more accurateconcentration level measurements of one or more target gases in agaseous environment. Although the various novel approaches are describedbelow with reference to associated exemplary embodiments that illustratehow the individual novel approaches provide improved gas sensingmethods, it is understood that two or more of the novel approaches canbe combined to provide gas sensing methods that represent a substantialimprovement over conventional gas sensing approaches.

According to a first novel approach, the core methodology mentionedabove is modified to facilitate thermal-reaction-type gas sensingapplications in which the one or more MTJ elements are utilized tocapture target gas concentration level data at high reactiontemperatures for later readout at lower (e.g., room) temperatures,thereby avoid the high-temperature resistance measurement problemsassociated with conventional thermal-reaction-type semiconductor gassensors. The first novel approach utilizes a thermal-reaction-type gassensing element that is configured to generate reaction heat in anamount proportional to a currently measured target gas concentrationlevel, and is operably thermally coupled to the MTJ element(s) such thatreaction heat generated by the gas sensing element at least partiallycontributes to each MTJ element's temperature. In accordance with anaspect of the first novel approach, each MTJ element is configured suchthat a storage blocking temperature of the MTJ element's storage layeris within a normal operating temperature range of thethermal-reaction-type gas sensor (i.e., with the high end of theoperating range defined by the reaction heat generated by thethermal-reaction-type gas sensing element), whereby the storage magneticorientation of the storage layer is made switchable during normaloperation of the gas sensor between two directions (i.e., whenever theMTJ element's temperature is above the storage blocking temperature). Inaddition, the MTJ element is further configured such that a referencemagnetic orientation of the MTJ element's reference layer remainspermanently fixed in one direction during normal operation of the gassensor, whereby the storage magnetic orientation is switchable between aparallel direction and an anti-parallel direction relative to thereference magnetic orientation. Consistent with conventional MTJelements, each MTJ element's resistance state has a low resistance valuewhen the storage magnetic orientation is parallel to the referencemagnetic orientation, and has a high resistance value when the storagemagnetic orientation is anti-parallel to the reference magneticorientation. As such, when subjected to a magnetic biasing force havinga direction opposite to its current magnetic direction, each MTJelement's resistance state is caused to switch from a first (e.g., low)resistance value to a second (e.g., high) resistance value when the MTJelement's temperature is increased above its storage blockingtemperature, and becomes fixed in the second (e.g., high) resistancevalue when the MTJ element's temperature subsequently decreases belowthe storage blocking temperature, whereby the switched/non-switchedstate can be determined by applying a fixed voltage across the MTJelement at a low (e.g., room) temperature, and measuring the resultingread current passing through the MTJ element in order to determine thepost-gas-sensing resistance state. Accordingly, by correlating thereaction heat generated by the gas sensing element with the storageblocking temperature of each MTJ element such that the MTJ element'stemperature increases above its storage blocking temperature when thegas sensing element is exposed to target gas above a predeterminedminimum concentration level, the first novel approach facilitatesthermal-reaction-type gas sensing applications in which MTJ elements areutilized to capture target gas concentration level data at high reactiontemperatures for later readout at lower (e.g., room) temperatures,thereby avoiding the high-temperature resistance measurement problemsassociated with conventional thermal-reaction-type semiconductor gassensors.

According to a first exemplary embodiment utilizing the first novelapproach mentioned above, in order to cause the MTJ element's resistancestate to reliably switch (e.g., from low to high) resistance values whenthe gas sensor is exposed to target gas above the predetermined minimumconcentration level, the MTJ element's resistance state is initialized(fixed) in a first (e.g., low) resistance state during a reset phase byway of temporarily increasing the MTJ's temperature above the MTJelement's storage blocking temperature while applying an initial (first)magnetic bias force (e.g., by way of generating an external magneticfield directed in a first direction), and then, during a subsequent gassensing phase, subjecting the MTJ element an opposing magnetic biasingforce (e.g., an anti-parallel alignment of magnetization vectors (AMV)force, spin torque transfer and/or an external magnetic field directedopposite to the first direction) while receiving reaction heat from thegas sensing element. In an exemplary embodiment, the reset phaseinvolves generating a first amount of control heat (e.g., by way ofactivating a resistive heating element) such that the MTJ temperatureincreases above the storage blocking temperature, generating an initialexternal magnetic bias field such that storage magnetic orientation isbiased into in the desired parallel direction relative to the referencemagnetic orientation, and then decreasing the MTJ temperature below thestorage blocking temperature (e.g., by de-activating the heating elementor otherwise acting to cool the MTJ element), whereby the storagemagnetic orientation becomes fixed in the initial (parallel) direction.Note that the initial external magnetic bias field is maintained duringthe cooling process, but may be terminated after the MTJ temperaturefalls below the storage blocking temperature. To facilitate CDS readoutoperations, an initial resistance value of the MTJ element is determinedat the end of the reset process, preferably after the MTJ element hascooled to room temperature, for example, by coupling the MTJ element toa read voltage source and measuring the resulting read current passingthrough the MTJ element.

Once the MTJ element's resistance state is initialized (fixed) in thefirst (e.g., low) resistance state during the reset phase, the presenceof target gas above (i.e., equal to or greater than) the predeterminedminimum concentration level is reliably detected by way of increasingthe MTJ element's temperature to and maintaining the MTJ element'stemperature at a work point temperature, which is below the storageblocking temperature, while a (second/opposing) magnetic bias force isapplied to the MTJ element. In one embodiment, the work pointtemperature generally coincides with the lowest temperature at whichreaction heat is generated by a gas sensing element in response to thetarget gas, and the temperature difference between the work pointtemperature and the MTJ element's storage blocking temperaturecorresponds to the predetermined minimum gas concentration level atwhich the target gas causes the MTJ element to switch resistance states.That is, the gas sensor is configured such that any reaction heatgenerated by the gas sensing element increases the MTJ element'stemperature above the work point temperature. Target gas concentrationsbelow the predetermined minimum gas concentration level produce reactionheat in a relatively low amount that fails to increase the MTJ element'stemperature above the MTJ element's storage blocking temperature, andtherefore fails to cause a change in the MTJ element's resistance state.Conversely, when the gas sensing element is exposed to target gas at orabove the predetermined minimum concentration level, gas sensing elementgenerates reaction heat in an amount sufficient to increase the MTJelement's temperature from the work point temperature to a temperatureabove (i.e., equal to or greater than) the MTJ element's storageblocking temperature. In one embodiment, maintaining the MTJ temperatureat or above the work point temperature involves generating acorresponding (second) amount of control heat (e.g., using the gassensor's heating element). In this case, when a currently measured gasconcentration level of the target gas is above the predetermined minimumgas concentration level, a corresponding amount of reaction heatgenerated by the gas sensing element combines with the corresponding(second) amount of control heat to increase the MTJ element'stemperature above its storage blocking temperature, whereby the secondmagnetic bias force (e.g., the AMV force or spin torque transfermentioned above, an external magnetic field having a direction oppositeto the initial direction, or a combination of two or more of theseforces) biases storage magnetic orientation into the anti-parallel(second) direction relative to the reference magnetic orientation,whereby the resistance state of the MTJ element switches from theinitial low resistance value to a high resistance value. The presence oftarget gas above the predetermined minimum gas concentration level isthus determined, for example, by way of identifying (e.g., byre-coupling the MTJ element to the read voltage source, measuring theresulting read current, and comparing the measured read current with theread current that was measured at the end of the reset phase) that afinal resistance state of the MTJ element had switched during the gassensing phase. Of course, if the final resistance state had notswitched, the initial and final resistance states would be the same. Ineither case, measured gas concentration level data indicating theswitched/non-switched state is generated indicating that target gas waseither blow or above the predetermined minimum gas concentration level.

According to a second novel approach, a thermal-reaction-typesemiconductor gas sensor includes multiple MTJ elements of the typedescribed above that are operably thermally coupled to one or morethermal-reaction-type gas sensing elements, where each of the MTJelements is configured with a different storage blocking temperature,thereby providing “higher resolution” gas concentration measurementsthan is achievable using a single MTJ element. That is, when two MTJelements are produced with two different storage blocking temperatures(i.e., either by way of fabrication techniques or because of inherentblocking temperature variances), when operably thermally coupled toreceive reaction heat from a thermal-reaction-type gas sensing elementssuch the MTJ temperatures of the two MTJ elements increase at the samerate, each of the two MTJ elements is caused to switch its resistancestate in response to a different gas concentration level. For example,in a hypothetical two-element gas sensor arrangement in which thestorage blocking temperature of a first MTJ element is lower than thestorage blocking temperature of the second MTJ element, only the firstMTJ element switches when the gas sensing element generates a relativelylow amount of reaction heat in response to a relatively low gasconcentration level that increases the MTJ temperatures of both MTJelements to the lower storage blocking temperature, and both MTJelements switch when the gas sensing element generates a relatively highamount of reaction heat in response to a relatively high gasconcentration level that increases the MTJ temperatures of both MTJelements from a work point temperature to a temperature equal to orgreater than the higher storage blocking temperature. In this two-MTJexample, the final resistance states of the two MTJ elements providesquantitative measured gas concentration level data indicating whetherthe measured gas concentration is in a range below lower concentrationlevel (indicated by neither MTJ element switching its resistance state),in a range between the lower and higher concentration levels (indicatedby switching of the first MTJ element and non-switching of the secondMTJ element), or in a range above the higher concentration level(indicated by both MTJ elements switching their resistance states). Byapplying this multiple-MTJ sensor approach to gas sensors including morethan two MTJ elements, each configured to switch its resistance state ata different gas concentration level, one skilled in the art can easilyrecognize that this arrangement can be used to quantitatively measuregas concentration levels with a high degree of accuracy (i.e., “highresolution”) by way detecting very small gas concentration leveldifferences. For example, a gas sensor would provide high resolutionquantitative gas measurement by including multiple MTJ elements that arerespectively configured to switch at equally-spaced different blockingtemperatures distributed over a given temperature range (e.g., atintervals of 5° C. over a range of 120° C. to 145° C., i.e., such that afirst MTJ element has a storage blocking temperature of 120° C., asecond MTJ element has a storage blocking temperature of 125° C., etc.).

According to an aspect of the second novel approach, a single (shared)field line is utilized to simultaneously control all of the MTJelements, thereby providing a simplified control methodology that isscalable to any number of MTJ elements. Similar to the first approachdescribed above, the field line structure is utilized to generate one ormore external magnetic fields that serve as a magnetic bias force duringa gas sensor's operating cycle (e.g., during the reset phase and/orduring the gas sensing phase). In accordance with the second novelapproach, the shared field line structure is operably magneticallycoupled to all of the multiple MTJ elements, and is configured such thatthe external magnetic field generated by the shared field line structuresimultaneously substantially equally biases the storage magneticorientations of all of the multiple MTJ elements in a single directiondefined by the external magnetic field. In a preferred embodiment, theshared field line arrangement is utilized during the reset phase to fixthe storage magnetic orientations of all associated MTJ elements in theparallel magnetic direction (i.e., in their low resistance state),whereby the MTJ elements are configured such that AMV force can beutilized as an applied magnetic bias force during the subsequent gassensing phase to cause each MTJ element to switch to the high resistancestate when its MTJ temperature is equal to or exceeds its associatedstorage blocking temperature. By fixing all of the multiple MTJ elementsin the low resistance state at reset, the process of determining theswitched/non-switched state of the MTJ elements during the subsequentgas sensing phase is thus greatly simplified in that no field line orMTJ currents are required during the gas sensing phase. Moreover,because all of the MTJ elements are reset into the same direction, asingle (shared) field line (and associated control circuitry) can beused to reset all of the MTJ elements, no matter how many MTJ elementsare used.

Various alternatives to the second novel approach are described withreference to associated specific embodiments. For example, in one case aheating element is utilized to generate control heat in the mannerdescribed above in order to facilitate reset and gas sensing operationsof the thermal-reaction-type gas sensor. In addition, although utilizingspin torque transfer to generate switching during the gas sensing phasefacilitates low power operations and is therefore presently preferred,in an alternative possible embodiment the shared field line arrangementmay be utilized during the gas sensing phase to generate an oppositeexternal magnetic field (e.g., by way of reversing the field linecurrent direction), whereby the storage magnetic orientations of allassociated MTJ elements are reliably biased into the anti-parallelmagnetic direction (i.e., into their high resistance state). In anotherexemplary embodiment, the multiple MTJ elements are series-connected tosimplify readout operations by facilitating determining the resistancestates of all of MTJ elements using a single read current (i.e., insteadof separate read currents passed through each MTJ element individually).

According to a third novel approach, semiconductor gas sensors areutilized that include multiple MTJ elements and an associated gas sensorsimilar to those described above (i.e., wherein the gas sensor isconfigured such that each MTJ element switches its resistance state inresponse to a different gas concentration level), wherein the multipleMTJ elements are coupled in a way that forms a NAND-typeseries-connected string such that a total string resistance of theseries-connected string is collectively defined by (e.g., the sum of)the corresponding individual MTJ resistance values of the MTJ elements.In addition to providing quantitative gas concentration levelmeasurements by way of utilizing multiple MTJ elements that switchresistance states at different gas concentration levels (as explainedabove), the NAND-type series-connected string arrangement furthersimplifies the gas sensing operation by way facilitating the use of asingle read current to determine the resistance state of all of theseries connected MTJ elements. That is, by initializing all of the MTJelements during the reset phase to a common (e.g., low) resistancestate, and controlling the gas sensing operation such that the MTJelements switch to the opposite (e.g., high) resistance state inaccordance with an associated gas concentration level, the number of MTJelements that switched state during a given gas sensing phase can bedetermined by calculating the difference between a measured final stringresistance and a stored initial string resistance value, where zerodifference would mean no MTJ elements switched states (indicated littleor no target gas), and a maximum possible difference would mean all ofthe MTJ elements switched states (indicating a high target gasconcentration level). Further, gas sensors implementing the NAND-typeseries-connected string arrangement are readily scalable to provide arange of measurement accuracies, for example, by way of increasing thenumber of MTJ elements in each series-connected string or connectingmultiple series-connected strings in parallel, and/or by configuringeach MTJ element of a series connected string to switch its resistancestate at a slightly different gas concentration level (e.g., by way offorming different MTJ elements with different lateral sizes), therebyproviding gas sensors capable of measuring very small gas concentrationlevel variations.

According to an aspect of the third novel approach, gas concentrationlevel measurement is conducted in accordance with correlated doublesampling (CDS) techniques by way of determining an initial resistancevalue of each NAND-type series-connected string at the end of the resetphase, conducting a gas sensing phase in the manner described above,determining a final resistance value of the series-connected stringafter the gas sensing phase is completed, and then utilizing adifference between the initial resistance value and the final resistancevalue to generate measured gas concentration level data. The applicationof CDS readout techniques to gas measurement is believed to be novel inthat it is not believed to be utilized in any commercially implementedgas sensing methodologies. In this case, the implementation of CDSreadout is made possible because each MTJ element's initial and finalresistance states are stably fixed (i.e., stored in a non-volatilemanner), and the accuracy provided by CDS readout facilitates the use ofa large number of MTJ elements in each series-connected string, which isturn facilitates highly accurate quantitative gas measurements in a veryshort amount of time (i.e., highly accurate quantitative gas measurementdata is generated using a single gas sensor operating cycle). Moreover,in the specific case of thermal-reaction-type gas sensors, the CDSreadout approach provides an additional benefit in that the stably fixed(non-volatile) data stored on the MTJ elements can be readout after theMTJ elements have cooled to a low (e.g., room temperature), wherebythermal-reaction-type gas sensors implementing the NAND-typeseries-connected string arrangement achieve substantially higher readoutaccuracies in comparison to conventional thermal-reaction-type gassensors that require measuring currents/resistances using MOSFETs duringthe gas sensing phase at high temperatures. That is, performing of bothread current measurements at room temperature represents a keydistinction of the present invention over all prior art gas sensingapproaches because this approach avoids high leakage currents and noise(i.e., due to resistance fluctuations connected with the fluctuations oftemperature) that limit the accuracy of conventional gas sensingmethodologies, and the use of CDS techniques even further increases theaccuracy of the measurement results. Although these and other benefitsof utilizing the NAND-type string and CDS readout approach aresignificantly greater when applied to thermal-reaction-type gas sensors,other semiconductor gas sensor types (e.g., chemical-reaction-type gassensors) may be produced that would also benefit from the NAND-typestring and CDS readout approach. Therefore, unless otherwise specified,the combined series-connected string and CDS readout of the third novelapproach is intended to apply to either chemical-reaction-type orthermal-reaction-type gas sensors unless thermal-reaction-type sensingelements are specified in the appended claims.

Various alternatives to the third novel approach are described withreference to associated specific embodiments. For example, heatingelements and field lines are utilized in accordance with techniquesdescribed above to facilitate room temperature readout operations and tofacilitate reliable switching. In another exemplary embodiment, measuredgas concentration level data is generated by way of calculating (e.g.,using a processor provided on the gas sensor device) a differencebetween the initial and final string resistance values, and theutilizing the calculated difference to access a corresponding gasconcentration level data stored on the gas sensor device (e.g., in alook-up table), and then transmitting the accessed corresponding gasconcentration level data through an output port of the gas sensor devicefor use in an external system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a simplified perspective view showing a semiconductor gassensor including an MTJ element according to an embodiment of thepresent invention;

FIG. 2 is flow diagram showing an exemplary operating cycle performed bythe gas sensor of FIG. 1 according to another embodiment of the presentinvention;

FIGS. 3(A), 3(B), 3(C), 3(D), 3(E) and 3(F) are simplified side viewsshowing the gas sensor of FIG. 1 during various operating phases of theoperating cycle shown in FIG. 2;

FIG. 4 is a simplified cross-sectional side view showing a membrane-typesemiconductor gas sensor forming using CMOS fabrication techniquesaccording to an exemplary embodiment of the present invention;

FIG. 5 is a simplified perspective view showing a semiconductor gassensor including multiple MTJ elements according to another exemplaryembodiment of the present invention;

FIG. 6 is a simplified side views showing a thermal-reaction-type gassensor including multiple MTJ elements according to another exemplaryembodiment of the present invention;

FIGS. 7(A), 7(B) and 7(C) are simplified side views showing the gassensor of FIG. 6 during different gas sensing phases;

FIG. 8 is resistance versus temperature graph illustrating operatingprinciples of the thermal-reaction-type gas sensor of FIG. 6;

FIG. 9 is simplified diagram showing multiple MTJ element stringsconnected in parallel according to another embodiment of the presentinvention; and

FIG. 10 is a simplified cross-sectional side view showing asemiconductor gas sensor including multiple string-based membrane-typesensor regions according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to an improvement in methods utilized todetect and measure selected target gases in an environment. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention as provided in the context of aparticular application and its requirements. As used herein, directionalterms such as “upper” and “lower” are intended to provide relativepositions for purposes of description, and are not intended to designatean absolute frame of reference. In addition, the terms “coupled” and“connected”, which are utilized herein, are defined as follows. The term“connected” is used to describe a direct connection between two circuitelements, for example, by way of a metal line formed in accordance withnormal integrated circuit fabrication techniques. In contrast, the term“coupled” is used to describe either a direct connection or an indirectconnection between two circuit elements. For example, two coupledelements may be directly connected by way of a metal line, or indirectlyconnected by way of an intervening circuit element (e.g., a capacitor,resistor, inductor, or by way of the source/drain terminals of atransistor). Various modifications to the preferred embodiment will beapparent to those with skill in the art, and the general principlesdefined herein may be applied to other embodiments. Therefore, thepresent invention is not intended to be limited to the particularembodiments shown and described, but is to be accorded the widest scopeconsistent with the principles and novel features herein disclosed.

FIG. 1 shows a thermal-reaction-type gas sensor 100 for detecting atarget gas G in an environment E according to a first exemplaryembodiment of the present invention. Gas sensor 100 includes a magnetictunnel junction (MTJ) element 111 that is operably thermally coupled toa gas sensing element 150 such that an MTJ temperature T₁₁₁ of MTJelement 111 is at least partially influenced by reaction heat H₁₅₀generated by gas sensing element 150. Although not shown in FIG. 1, itis understood that an intervening structure (e.g., a layer of dielectricmaterial) is disposed between gas sensing element 150 and MTJ element111.

MTJ element 111 generally includes a reference layer 120 and a storagelayer 130 separated by a tunnel dielectric layer 114. Reference layer120 includes a magnetic structure that defines a reference magneticorientation M₁₂₀, and storage layer 130 includes a second magneticstructure that defines a storage magnetic orientation M₁₃₀. According toan aspect of the present invention, reference layer 120 is configured asa high-coercivity magnetic structure such that reference magneticorientation M₁₂₀ remains fixed in a single magnetic direction duringnormal operation of gas sensor 100, and storage layer 130 is configuredas a low-coercivity magnetic structure such that storage magneticorientation M₁₃₀ is switchable (changeable) between two magneticdirections during normal operation of gas sensor 100, whereby aresistance state R₁₁₁ of MTJ element 111 is changeable (switchable)between two (i.e., relatively low and relatively high) resistancevalues. In the exemplary embodiment, reference layer 120 is configuredsuch that reference magnetic direction M₁₂₀ remains fixed in firstmagnetic direction MD₀, which is depicted using right-pointing arrow inFIG. 1, and storage layer 130 is configured such that storage magneticorientation M₁₃₀ is switchable between first magnetic direction MD₀ andsecond magnetic direction MD₁, which is depicted using left-pointingarrow in FIG. 1 (i.e., opposite to magnetic direction MD₀). The magneticorientations of reference layer 120 and storage layer 130 are “parallel”when storage magnetic orientation M₁₃₀ is fixed in the same direction asreference magnetic direction M₁₂₀ (e.g., both storage magneticorientation M₁₃₀ and reference magnetic direction M₁₂₀ are aligned infirst magnetic direction MD₀). Conversely, the magnetic orientations ofreference layer 120 and storage layer 130 are “anti-parallel” whenstorage magnetic orientation M₁₃₀ is fixed in an opposite direction tothat of reference magnetic direction M₁₂₀ (e.g., storage magneticorientation M₁₃₀ is fixed in second magnetic direction MD₁ and referencemagnetic direction M₁₂₀ is fixed in first magnetic direction MD₀). Acurrent resistance state R₁₁₁ of MTJ element 111 is correlated to theparallel/non-parallel directions in that resistance state R₁₁₁ has a lowresistance value when storage magnetic orientation M₁₃₀ is parallel toreference magnetic orientation M₁₂₀, and has a high resistance valuewhen storage magnetic orientation M₁₃₀ is anti-parallel/opposite toreference magnetic orientation M₁₂₀. For reference, typical resistancesof high and low states for an MTJ having a diameter of 200 nm and MgOthickness of 12-14 A are 2 kOhm and 700 Ohm to 1 kOhm, respectively,which corresponds to a tunnel magneto-resistance ratio (TMR) ofapproximately 100% to 200%. Accordingly, the parallel/antiparallelorientation of storage layer 130 relative to reference layer 120 at agiven point in time can be easily determined by way of determiningresistance state R₁₁₁ of MTJ element 111 at that time.

According to an aspect of the first exemplary embodiment, storagemagnetic orientation M₁₃₀ is made switchable between parallel andanti-parallel directions relative to reference magnetic orientation M₁₂₀by way of configuring storage layer 130 such that its associated storageblocking temperature T_(B130) that is within the normal operatingtemperature range experienced by MTJ element 111 (i.e., the temperaturerange of MTJ temperature T₁₁₁), and by way of configuring referencelayer 120 such that it has an associated reference blocking temperatureT_(B120) that is higher than the normal operating temperature rangeexperienced by MTJ element 111. The normal operating temperature rangeof MTJ temperature T₁₁₁ is established by temperatures generated on gassensor 100 under normal operating conditions, whereby MTJ temperatureT₁₁₁ of MTJ element 111 normally varies between a minimum temperature(e.g., room temperature, approximately 24° C.) and a maximum temperature(e.g., approximately 250° C.). In one embodiment, storage layer 130 is amulti-layer structure including an antiferromagnetic (AF) structure 132and a ferromagnetic structure 135 disposed in close proximity such thatan exchange interaction between the two structures produces storagemagnetic direction M₁₃₀, and the desired switchability of storagemagnetic orientation M₁₂₀ at normal operating temperatures of gas sensor100 is achieved by way of fabricating AF structure 132 using an AFmaterial having an associated storage blocking temperature T_(B130) thatis within the expected normal operating temperature range of MTJtemperature T₁₁₁. That is, when a temperature of AF structure 132 isbelow its associated storage blocking temperature T_(B130), it becomeshighly resistant to switching its magnetic direction in response to anexternal magnetic bias force, whereby storage magnetic direction M₁₃₀becomes “fixed” (i.e., pinned or unchangeable) in one of two directions(e.g., MD₀ or MD₁). Conversely, when the temperature of AF structure 132is above its associated storage blocking temperature T_(B130), AFstructure 132 becomes less resistant to change, whereby storage magneticorientation M₁₃₀ is switchable between directions MD₀ and MD₁ by appliedmagnetic bias forces (e.g., external magnetic fields). With storagelayer 130 configured in this way, storage magnetic orientation M₁₃₀ isswitchable during operation of gas sensor 100 whenever MTJ temperatureT₁₁₁ increases above storage blocking temperature T_(B130). In contrast,reference layer 120 is fabricated using a different AF structure 122disposed in contact with an associated ferromagnetic structure 125,where AF structure 122 is fabricated using an AF material having anassociated storage blocking temperature T_(B120) that is greater thanthe normal operating temperature range of MTJ element 111, wherebyreference magnetic orientation M₁₂₀ remains permanently fixed (e.g., infirst magnetic direction MD₀) because MTJ temperature T₁₁₁ of MTJelement 111 does not increase above reference blocking temperatureT_(B120) during normal operations of gas sensor 100.

According to a presently preferred embodiment illustrated in FIG. 1, MTJelement 111 is produced in a stack-type arrangement with (first)ferromagnetic structure 125 and (second) ferromagnetic structure 135respectively disposed in contact with opposite (upper and lower)surfaces of tunnel dielectric layer 114, and with (first) AF structure122 and (second) AF structure 132 respectively disposed on respectiveopposite (upper and lower) surfaces of ferromagnetic structures 125 and135. In a specific exemplary embodiment, ferromagnetic layers 125 and135 comprise one or more of Fe, Co, Ni and their alloys, such as, e.g.,FeCo. According to an aspect of the preferred embodiment, reference AFstructure 122 comprises a first AF material exhibiting associatedreference blocking temperature T_(B120) and storage AF structure 132comprises a second (different) AF material having associated storageblocking temperature T_(B130), where the two different AF materials areselected such that the reference blocking temperature T_(B120) issubstantially greater (higher) than the storage blocking temperatureT_(B130). More specifically, reference AF structure 122 comprises afirst AF material having a reference blocking temperature T_(B120) thatis preferably higher than the maximum expected operating temperature ofgas sensor 100 (e.g., greater than approximately 250° C.), wherebyreference magnetic orientation M₁₂₀ remains effectively permanentlyfixed (e.g., in direction MD₀ indicated in FIG. 1) after beinginitialized during fabrication. In contrast, storage AF structure 132comprises an AF material having a storage blocking temperature T_(B130)that is approximately midway within the expected normal operatingtemperature range of gas sensor 100 (e.g., approximately 125° C.). In aspecific exemplary embodiment, reference AF structure 122 comprises anAF material (e.g., one or more of PtMn or NiMn) having a referenceblocking temperature T_(B120) in the range of 250-350° C., and storageAF structure 132 comprises an AF material (e.g., one or more of FeMn orIrMn) having a storage blocking temperature T_(B130) in the range of120-250° C. In one embodiment, the AF material in each layer can becoupled to a synthetic AF structure comprising of two magnetic layerssandwiched with a thin ruthenium layer—such a synthetic AF structure hasa very strong antiparallel coupling. If the ferromagnetic with AFpinning is coupled also to a synthetic AF structure, the coupling of thereference ferromagnetic layer over the storage ferromagnetic layer issmall (synthetic AF structures produce small stray magnetic fieldsextending into the opposite electrode).

In an exemplary embodiment (referring to FIG. 1), MTJ element 111 has anoverall cell thickness L₁₁₁ in the range of 50 to 200 nanometers, andhas a nominal width/diameter W₁₁₁ (i.e., maximum top view dimension) inthe range of 50 to 500 nanometers, and more preferably in the range of100 to 250 nanometers. Reference AF structure 122 is fabricated with athickness L₁₂₂ in the range of 10 and 30 nm, reference ferromagneticstructure 125 is fabricated with a thickness L₁₂₅ in the range of 5 and70 nm, tunnel dielectric layer 114 (e.g., magnesium oxide (MgO) oraluminum oxide (Al₂O₃)) is fabricated with a thicknesses L₁₁₄ in therange of 10 to 20 Angstroms, storage ferromagnetic structure 135 isfabricated with a thickness L₁₃₅ in the range of 5 and 70 nm, andstorage AF structure 132 is fabricated with a thickness L₁₃₂ in therange of 5 and 30 nm. Forming MTJ element 111 using these dimensionsboth minimize fabrication defects (e.g., shorts) and produces desirableresistance characteristics that facilitate the write/program and compareoperations that are described below.

In one embodiment, MTJ element 111 is further configured such that ananti-parallel alignment of magnetization vectors (AMV) force or spintorque transfer STT (spin-polarized current) is directed from referencelayer 120 into the free magnetic layer (storage layer 130) withsufficient magnetic force to facilitate switching of storage magneticorientation M₁₃₀ during the operating cycle of gas sensor 100. In thecase of AMV, although magnetic fields typically do not extend outsideeach AFM/FM stack, it is possible to modify layer borders and grains ofthe AFM such that a magnetic field generated by reference layer 120imposes a magnetic bias on storage layer 120 having sufficient strengthto switch storage layer 120 from the parallel to the anti-parallelorientation. In the case of spin torque transfer (STT), an electricalcurrent in the MTJ causes spin torque that is transferred by electronsmoving from reference layer 120 to storage layer. Referring to FIG. 1,both force AMV and spin torque STT are directed from reference layer 120to storage layer 130, where a magnetic moment (indicated by thedouble-headed dashed-line arrow in FIG. 1) is transferred to storagelayer 130 by the bias force or spin torque of electrons polarized byreference layer 120. Both the AMV force and spin torque transfer resultin bias forces similar to the repelling force experienced by two barmagnets placed in parallel with their North poles together—in theabsence of a countering force, one of the magnets will rotate because aNorth-to-South pole alignment is the lower energy state. When thedimensions of MTJ element 111 are adjusted using known techniques tosufficiently enhance AMV, the resulting field provides a sufficientlystrong magnetic bias force that can be used to bias storage magneticorientation M₁₃₀ into the anti-parallel direction relative to referencemagnetic orientation M₁₂₀ when MTJ temperature T₁₁₁ is above storageblocking temperature T_(B130) (i.e., and below reference blockingtemperature T_(B120), and in the absence of a conflicting externalmagnetic biasing field). Specifically, when storage magnetic orientationM₁₃₀ and reference magnetic orientation M₁₂₀ are in parallel directions(e.g., referring to FIG. 1, both are fixed in first magnetic directionMD₀) and then MTJ temperature T₁₁₁ is increased above storage blockingtemperature T_(B130) (and no current is supplied to field line structure140), force AMV causes storage magnetic orientation M₁₃₀ to switch tothe anti-parallel (opposite) direction relative to reference magneticorientation M₁₂₀ (i.e., such that reference magnetic orientation M₁₂₀remains fixed in first magnetic direction MD₀, but storage magneticorientation M130 switches to second magnetic direction MD₁). ConfiguringMTJ element 110 to produce sufficient AMV force therefore provides anadvantage over STT and field line force F2 in that this arrangementfacilitates reliably switching resistance state R₁₁₁ of MTJ element 111from the low resistance (parallel direction) value to the highresistance (anti-parallel direction) value without applying any currentto gas sensor 100, thereby facilitating low energy consumptionoperations.

Referring again to FIG. 1, gas sensing element 150 is disposed on anexternal region of gas sensor 100 such that it physically contactsenvironment E (and, hence, target gas G, when present in environment E)during gas sensing operations, and is disposed relative to MTJ element111 such that reaction heat H₁₅₀ generated by gas sensing layer 150significantly influences MTJ temperature T₁₁₁ of MTJ element 111. In oneembodiment, gas sensing element 150 is separated from by a thin layer ofpassivation material (not shown), and is configured in a manner similarto thermal-reaction-type gas sensing elements used in conventionalthermal-reaction-type gas sensors such that reaction heat H₁₅₀ isgenerated by gas sensing element 150 in an amount proportion to anactual (currently measured) concentration level C of target gas G inenvironment E. In a presently preferred embodiment, gas sensing element150 is a combustion-type gas sensing element in which reaction heat H₁₅₀is generated by combustion of target gas G that comes into contact withgas sensing element 150 in a manner consistent with conventionalreaction type (ii), which is described in the background section above.In an alternative embodiment, gas sensing element 150 is implementedusing reaction type (iii) in which reaction heat H₁₅₀ is produced by wayof thermal conductivity of target gas G passing over gas sensing element150.

According to another aspect of the first embodiment, gas sensor 100 isconfigured such that reaction heat H₁₅₀ generated by gas sensing element150 during a gas sensing phase increases MTJ temperature T₁₁₁ of MTJelement 111 from a lower temperature above (i.e., equal to or greaterthan) storage blocking temperature T_(B130) only when currently measuredconcentration level C of target gas G in environment E is above (i.e.,equal to or greater than) a predetermined minimum concentration level.That is, because the amount of reaction heat H₁₅₀ generated by gassensing element 150 is proportional to a given measured concentrationlevel, because MTJ temperature T₁₁₁ of MTJ element 111 is proportionalto reaction heat H₁₅₀ by way of the operable thermal coupling betweengas sensing element 150 and MTJ element 111, and because storagemagnetic orientation M₁₃₀ resists switching directions (i.e., fromparallel to anti-parallel directions, or vice versa) unless MTJtemperature T₁₁₁ is above storage blocking temperature T_(B130), gassensor 100 is configured to indicate whether a currently measuredconcentration level C of target gas G is above or below thepredetermined minimum gas concentration level by way of theswitched/non-switched state of storage layer 130 after the gas sensingphase. For example, when a given measured concentration level C oftarget gas G were below the predetermined minimum concentration level,the generated amount of reaction heat H₁₅₀ would be insufficient toincrease MTJ temperature T₁₁₁ above storage blocking temperatureT_(B130), whereby switching of storage magnetic orientation M₁₃₀ wouldnot be possible, and gas sensor 100 would indicate that the givenmeasured concentration level is below the predetermined minimum gasconcentration level by way of the non-switched state of storage layer130 after completion of the gas sensing phase. Conversely, when a givenmeasured concentration level C of target gas G is above thepredetermined minimum concentration level, the generated amount ofreaction heat H₁₅₀ increases MTJ temperature T₁₁₁ above storage blockingtemperature T_(B130), whereby switching of storage magnetic orientationM₁₃₀ is made possible (i.e., by way of an applied magnetic bias force,as described below), and gas sensor 100 indicates that the givenmeasured concentration level is above the predetermined minimum gasconcentration level by way of the resulting switched state of storagelayer 130 after completion of the gas sensing phase. Thus, byconfiguring gas sensor 100 such that reaction heat H₁₅₀ increases MTJtemperature T₁₁₁ above storage blocking temperature T_(B130) only whencurrently measured concentration level C is above the predeterminedminimum concentration level, the present invention facilitates detectingtarget gas G in concentration levels above the predetermined minimum gasconcentration level by way of determining the switched/non-switchedstate of storage magnetic orientation M₁₃₀ after a given gas sensingphase of the gas sensor's operating cycle.

According to an exemplary specific embodiment depicted in simplifiedform in FIG. 1, the readout of resistance state R₁₁₁ of MTJ element 111is implemented by coupling MTJ element 111 between a read voltage sourceV₁₁₁ and a ground terminal by way of a select transistor 117, andcontrolling select transistor 117 by way of MTJ element measurementsub-circuit 171 during the readout phase (and, optionally, at the end ofthe reset phase) of each sensor operating cycle. More specifically,voltage source V₁₁₁ is applied on a conductive line 115-11 (e.g.,metallization line), which is coupled to storage AF layer 132 of MTJelement 111 by way of a metal via 116-1, and reference AF layer 122 ofMTJ element 111 is in turn connected by way of a metal via 116-2 and aconductive line 115-12 to ground by way of select transistor 117. Withthis arrangement, determining resistance state R₁₁₁ of MTJ element 111is achieved by turning on select transistor 117 to generate a readcurrent I₁₁₁ from fixed voltage source V₁₁₁ through MTJ element 111,measuring the resulting read current I₁₁₁ (e.g., using a current sensor172), and the determining the high/low resistance value of resistancestate R₁₁₁ by way of determining whether read current is relatively high(indicating resistance state R₁₁₁ is low) or relatively low (indicatingresistance state R₁₁₁ is high).

Referring again to FIG. 1, gas sensor 100 further includes an on-chipfield line structure 140 that is physically spaced from MTJ element 111by an intervening dielectric or insulating structure (not shown), isoperably magnetically coupled to storage layer 130 of MTJ element 111,and is configured to generate an external magnetic field F exerting amagnetic force that allows external magnetic field F to serve as themagnetic biasing force utilized to reset and/or switch storage magneticorientation M₁₃₀ during reset and/or read operating phases. In thedisclosed embodiment, field line structure 140 comprises an elongatedmetal structure that is coupled to a voltage source V₁₄₀ by a conductor115-21 and to a field line control sub-circuit 175 by way of a conductor115-22, and field line sub-circuit 175 is configured to actuate fieldline structure 140 to generate field F by way of generating a field linecurrent I₁₄₀ through field line structure 140, whereby a magneticdirection of the magnetic field F is changeable by way of changing theflow direction of field line current I₁₄₀. As described in additionaldetail below, this arrangement facilitates switching storage magneticorientation M₁₃₀ between the parallel and anti-parallel directionsrelative to reference magnetic orientation M₁₂₀ when MTJ temperatureT₁₁₁ is above storage blocking temperature T_(B130), and may be used ineither or both of the reset and readout phases of the sensor operatingcycle. Although on-chip field line 140 may be required for theoperations of gas sensor 100 described below, in some embodiments (notshown) suitable alternative magnetic bias forces that are utilizedduring the reset and/or gas sensing phases may be generated fromoff-chip sources, and in such embodiments field line 140 may be omitted.

Referring to the lower left portion of FIG. 1, according to anotherembodiment, gas sensor 100 further includes an on-chip resistive heatingelement 145 that is physically spaced from field line structure 140 byan intervening dielectric or insulating structure (not shown), isoperably thermally coupled to MTJ element 111 and to gas sensing element150, and is configured to generate control heat H₁₄₅. In the disclosedembodiment, heating element 145 comprises a coil structure that isformed using polycrystalline silicon or Tungsten that is coupled to avoltage source V₁₄₅ by a conductor 115-31 and to a heating elementcontrol sub-circuit 177 by way of a conductor 115-32, and heatingelement control sub-circuit 177 is configured to actuate heating element145 by way of generating a heater current I₁₄₅ from voltage source V₁₄₅through heating element 145. As set forth below, heating element controlsub-circuit 177 is configured to actuate heating element 145 during thereset and readout operating phases such that control heat H₁₄₅increases/decreases the temperatures of MTJ element 111 and gas sensingelement 150 to create desired operating temperatures on gas sensor 100.In one embodiment, the actuation of heating element 145 is controlledusing a temperature sensor 147 that is coupled between a voltage sourceV₁₄₇ and heating element control sub-circuit 177, and is configuredusing known techniques such that a temperature sensor control currentI₁₄₇ passing through temperature sensor 147 provides heating elementcontrol sub-circuit 177 with real-time temperature data usable forcontrolling heating element 145 to generate the desired operatingtemperatures. Although on-chip heating element 145 may be required forthe operations of gas sensor 100 described below, in some embodiments(not shown) the required control heat may be generated from off-chipheat sources, and in such embodiments heating element 145 may beomitted.

FIG. 2 is a flow diagram depicting an exemplary sensor operating cycleimplemented by gas sensor 100 (FIG. 1) during normal operatingconditions according to another embodiment of the present invention. Asindicated along the left edge of FIG. 2, the sensor operating cycle isgenerally divided into a reset phase, a gas sensing phase, a readoutphase, and an optional output processing (OP) phase that are performedsequentially as indicated by the TIME arrow located along the left edgeof FIG. 2. Also included in FIG. 2 are references to the varioussub-circuits of control circuit 170 (FIG. 1) that function as describedbelow during the various phases of the operating cycle. In addition,exemplary operation cycle phases are described with reference to FIG. 2are depicted with reference to gas sensor 100 in a simplified form inFIGS. 3(A) through 3(F), wherein the notation “t[xy]” next to thereference number “100” denotes relative time and alternative operatingconditions. For example, FIG. 3(A) depicts gas sensor 100 at an initialtime t1, which is indicated by the notation “100(t 1)”, and FIGS. 3(C)and 3(D) depict gas sensor 100 at a subsequent time t3 under twodifferent operating conditions, which are indicated by the notations“100(t 3 a)” and “100(t 3 b)”. Similarly, various signals and magneticorientations include the time-indicating suffix “-x” indicating a stateof the signal/orientation at the associated time period (e.g., MTJtemperature T₁₁₁₋₁ in FIG. 3(A) denotes MTJ temperature T₁₁₁ at timet1). The time sequence notation is intended merely to denote therelative operation sequence, and is not intended to denote a uniformtime period between each depicted operation process. Note that, asdescribed below, the present invention is preferably implemented usingmultiple series-connected MTJ elements in order to produce highresolution quantitative gas concentration level information. As such,although the sensor operating cycle depicted in FIG. 2 is described in asimplified form with reference to single-MTJ-element gas sensor 100, themethodology set forth in FIG. 2 is understood to also apply to gassensors utilizing multiple series-connected MTJ elements.

Referring to the upper left portion of FIG. 2, the sensor operatingcycle begins with the reset phase, which functions to initialize (fix)the resistance state of MTJ element 111 in an initial (first) resistancevalue. Note that the exemplary embodiment implements a low initialresistance state (i.e., parallel magnetic orientation between thereference and storage layers) to facilitate using AMV or STT to causeswitching during the subsequent gas sensing phase, which is preferredfor reasons set forth below, but an initial high resistance state (i.e.,anti-parallel magnetic orientation) may also be used, but this wouldrequire an applied external field to generate switching during the gassensing phase.

Referring to the upper left portion of FIG. 2, the reset phase includesincreasing MTJ temperature T₁₁₁ above storage blocking temperatureT_(B130) (block 211), applying a magnetic bias force having the desiredinitial magnetic direction (block 213), then decreasing MTJ temperatureT₁₁₁ below storage blocking temperature T_(B130) (block 215). Thisthree-stage process for fixing storage magnetic orientation M₁₃₀ in thedesired initial (parallel) direction may be accomplished using varioustechniques. In the exemplary (and presently preferred) embodimentindicated in FIG. 2, the heating/cooling processes (blocks 211 and 215)are positioned under the heading HEATING ELEMENT CONTROL SUB-CIRCUIT 177to indicate that these processes are implemented by heating element 145of gas sensor 100 (FIG. 1), and the applied bias force process (block213) is positioned under the heading FIELD LINE CONTROL SUB-CIRCUIT 175to indicate that the applied bias force is generated by field linestructure 140 of gas sensor 100 (FIG. 1). As indicated in FIG. 3(A), therequired control heat H₁₄₅₋₁ is generated by activating heating element145 using an applied heater current I₁₄₅₋₁, and the required magneticbias force is generated by applying a field line current I₁₄₀₋₁ to fieldline structure 140 such that it generates a (first) external magneticfield F1 having the desired magnetic direction (indicated by the curveddashed-line arrow pointing left in FIG. 3(A)). Subsequent cooling of MTJelement 111 below storage blocking temperature T_(B130) is implementedin the exemplary embodiment by de-activating heating element 145 (i.e.,terminating heater current I₁₄₅₋₁), whereby heat transfer from MTJelement 111 to adjacent cooler structures causes MTJ temperature T₁₁₁ todecrease. In alternative embodiments (not shown), other heat sources(e.g., adjacent MTJ elements) may be utilized to generate the requiredheat energy, and the cooling process may be enhanced, by way of heatsink structures. Moreover, applying the required magnetic bias force maybe implemented using other structures and/or applied in a directionopposite to the exemplary (parallel) direction. For example, a spintorque transfer generated by reference layer 120 may be used to fixstorage magnetic orientation M₁₃₀ in an initial anti-parallel directionrelative to reference magnetic orientation M₁₂₀. In view of theseoptions, it is understood the appended claims are not limited to theexemplary embodiment unless otherwise specified.

After the storage magnetic orientation is fixed in its initial direction(i.e., after MTJ temperature T₁₁₁₋₁ falls below storage blockingtemperature T_(B130) per block 215), the initial magnetic bias forceused to set storage magnetic orientation M₁₃₀ in the initial directionis no longer needed. Referring again to the central column of FIG. 2,because this initial magnetic bias force is generated by way ofactivating the field line structure to generate external magnetic fieldF (block 213), terminating the magnetic bias force is implemented byde-activating the field line structure, which is depicted in FIG. 3(B)by way of the absence of a field line current applied to field linestructure 140. In the case where spin torque transfer is used to resetthe storage magnetic orientation, terminating this force is notpossible, so the terminate process of block 216 is considered optional.

Referring block 217 on the right side of FIG. 2, in one embodiment aninitial resistance value R₁₁₁₋₂ of MTJ element 111 is determined at theend of the reset phase to facilitate correlated double sampling (CDS)readout operations. Note that block 217 is located under the heading MTJELEMENT CONTROL SUB-CIRCUIT 171/172 to indicate that this process isimplemented by way of utilizing MTJ element measurement circuit 171 andcurrent sensor 172 (both shown in FIG. 1 and described above). FIG. 3(B)depicts gas sensor 100(t 2) during an exemplary initial readout processduring which MTJ element 111 is coupled to a fixed read voltage (i.e.,V₁₁₁, shown in FIG. 1), and the resulting read current I₁₁₁₋₂ passingthrough MTJ element 111 is measured that indicates initial resistanceR₁₁₁₋₁. Note that, because storage magnetic orientation M₁₃₀₋₂ remainsfixed in the initial parallel direction relative to reference magneticorientation M₁₂₀, initial resistance R₁₁₁₋₁ of MTJ element 111 has arelatively low resistance value (indicated as “R_(MIN)” in FIG. 3(B)).In addition, according to a presently preferred embodiment, the initialreadout process is conducted after MTJ element has cooled substantiallyto room temperature (indicated in FIG. 3(B) by “T₁₁₁₋₂=RT”) beforemeasuring read current I₁₁₁₋₂.

As indicated along the left side of FIG. 2, the gas sensing phase isperformed at the end of the resent phase. The gas sensing phasegenerally involves maintaining MTJ element 111 at a temperature belowstorage blocking temperature T_(B130) while applying a magnetic biasforce corresponding to the anti-parallel direction of the reference andstorage layer magnetizations in the MTJ element, where gas sensor 100 isconfigured such that the presence of target gas G above (i.e., equal toor greater than) a predetermined minimum concentration level will causegas sensing element 150 to generate sufficient reaction heat H₁₅₀ toraise MTJ temperature T₁₁₁ above storage blocking temperature T_(B130),thereby causing storage magnetic orientation M₁₃₀ to switch from theparallel to the anti-parallel direction. As discussed below, each of thetwo operating parameters required to affect the gas sensing phase (i.e.,proper MTJ temperature and applied magnetic bias field) may be achievedby alternative methodologies, and as such the illustrated examplesdescribed below are not intended to be limiting.

Referring to FIG. 2 and to FIGS. 3(A) and 3(B), in the illustratedembodiment maintaining MTJ element 111 at a temperature slightly belowstorage blocking temperature T_(B130) in the illustrated embodiment isachieved by actuating heating element 145 to generate control heatH₁₄₅₋₃ in an amount that increases (i.e., when starting from a lowertemperature) and maintains MTJ temperature T₁₁₁₋₃ at an optimalpredetermined gas sensing, which is referred to herein as a work pointtemperature. As explained in additional detail below with reference toFIG. 8, the work point temperature of a given sensor arrangementcoincides with the lowest temperature at which reaction heat H₁₅₀₋₃ isgenerated by gas sensing element 150 in response to target gas G, andstorage blocking temperature T_(B130) of MTJ element 111 is set anamount above the work point temperature such that gas sensing element150 generates an amount of reaction heat H₁₅₀₋₃ that increases MTJtemperature T₁₁₁₋₃ from the work point temperature to storage blockingtemperature T_(B130) when target gas G is present at the predeterminedminimum concentration level. By way of illustrative example, FIGS. 3(C)and 3(D) show two gas alternative sensing phases, with gas sensor 100(t3 a) in FIG. 3(C) disposed in an environment E1 including target gas Gat a relatively low gas concentration level C1, and gas sensor 100(t 3b) in FIG. 3(D) disposed in an environment E2 including target gas G ata relatively high gas concentration level C2. In both cases, heatingelement 145 receives the same heater current I₁₄₅₋₃ such that the sameamount of control heat H₁₄₅₋₃ is generated in both FIGS. 3(C) and 3(D).In the case illustrated in FIG. 3(C), because currently measured gasconcentration level C1 of target gas G is below the predeterminedminimum gas concentration level, the combination of reaction heatH_(150-4a) generated by gas sensing element 150 and control heat H₁₄₅₋₃generated by heating element 145 fails to increase MTJ temperatureT_(111-3b) from the work point temperature to storage blockingtemperature T_(m30), whereby storage magnetic orientation M_(130-3a)remains fixed (i.e., remains in the initial parallel direction relativeto reference magnetic orientation M₁₂₀), and resistance state R₁₁₁ ofMTJ element 111 retains its initial low resistance value. In contrast,as illustrated in FIG. 3(D), when currently measured gas concentrationlevel C2 of target gas G is above the predetermined minimum gasconcentration level, reaction heat H_(150-4b) generated by gas sensingelement 150 combines with control heat H₁₄₅₋₃ to increase MTJtemperature T_(111-3b) from the work point temperature to a temperatureabove storage blocking temperature T_(B130), thereby unpinning storagelayer 130 such that magnetic bias force AMV/STT/F2 biases storagemagnetic orientation M_(130-3b) into the anti-parallel (second)direction relative to reference magnetic orientation M₁₂₀, wherebyresistance state R₁₁₁ of MTJ element 111 switches from the initial lowresistance value to a high resistance value. The work point temperatureis generally determined by heating element 150 and the target gas type(i.e., for a given gas sensor, the work point temperature changes fordifferent target gasses). Similarly, as discussed in additional detailherein, the storage blocking temperature T_(B130) for a given MTJelement 111 is generally fixed (unchangeable) after fabrication, but maybe influenced during fabrication by random factors (e.g., crystal grainsize) and different design parameters (e.g., lateral size and layerthickness). Therefore, the predetermined gas concentration level for gassensor 100 is the target gas concentration level at which heatingelement 150 generates the amount of reaction heat H₁₅₀ required toincrease MTJ temperature T₁₁₁ from the work point temperature to storageblocking temperature T_(B130), and therefore is adjustable either by wayof altering heating element 150 (i.e., configuring heating element 150such that the work point temperature is set at an optimal temperaturefor a given storage blocking temperature), or by altering MTJ element111 (i.e., by configuring MTJ element 111 such that storage blockingtemperature T_(B130) occurs at an optimal temperature for a given workpoint temperature).

Although increasing/maintaining MTJ element 111 at the work pointtemperature involves actuating on-chip heater 145 in a preferredembodiment, achieving/maintaining the work point temperature may beachieved using other methods. For example, in cases where the work pointtemperature may be room temperature, no control heat would be needed tomaintain MTJ element at room temperature. In other cases, an off-chipheat source may be used to maintain gas sensor 100 at the work pointtemperature. Accordingly, unless specified in the appended claims,maintaining MTJ element 111 at the work point temperature during the gassensing phase should not be limited to generating control heat using anon-chip heating element such as heating element 145.

Turning to the applied magnetic bias force parameter that is required toperform the gas sensing phase, as illustrated in FIGS. 3(C) and 3(D), abenefit of initializing storage layer in the low resistance state isthat AMV (discussed above) could be utilized to provide the appliedmagnetic bias force during the gas sensing phase. In this case, theexchange interaction of the reference and storage layers serves thefunction of the external magnetic force. MTJ elements are switched intothe high resistance state (antiparallel orientations of storage andreference layer magnetizations) due to the magnetic interaction of thereference layer with the storage layer. This benefits gas sensingoperation because it allows gas sensor 100 to perform gas sensing atzero operating voltages (i.e., other than the heater currents) by way ofeliminating the need for an externally generated magnetic bias force.Switching of the storage layer can be facilitated by using externalmagnetic fields at the stage of sensing (if the exchange bias from thereference is not sufficient to switch the storage layer). In this case,e.g., spin torque transfer can be used. Alternatively, as indicated byblock 223 in FIG. 2 and in FIGS. 3(C) and 3(D), in the case where AMVand STT are not available in sufficient strength to produce reliableswitching, the magnetic bias force applied to MTJ element 111 during thegas sensing phase may be applied in the form of a (second) externalmagnetic field F2 generated by way of passing a field line currentI₁₄₀₋₃ in a direction opposite to reset field line current I₁₄₀₋₁ (seeFIG. 3(A)), whereby external magnetic field F2 biases storage layer 130in the anti-parallel direction to facilitate switching of MTJ element130 when the measured target gas concentration is above thepredetermined minimum concentration level. In yet another embodiment,two or more of AMV, spin torque transfer STT and external magnetic fieldF2 are utilized in combination to produce a reliable magnetic bias forceduring the gas sensing phase.

Referring to the lower portion of FIG. 2, a readout phase of the sensoroperating cycle is performed after completion of the gas sensing phase.As indicated in block 231, in cases where control heat is used tomaintain MTJ element 111 at the work point temperature, the readoutphase begins by terminating the production of control heat, therebyallowing MTJ temperature to drop to a low temperature that precludesfurther detection of the target gas. In cases where external magneticfield F2 is utilized during the gas sensing phase, it is then terminated(block 233). The presence of target gas G above the predeterminedminimum gas concentration level is then determined, for example, by wayof reading the final resistance state of MTJ element 111 (block 217).Referring to FIGS. 3(E) and 3(F), in one embodiment this readout processis performed by re-coupling MTJ element 11 to the fixed read voltagesource (i.e., voltage V₁₁₁, see FIG. 1), and then measuring theresulting read current I_(111-4x) (i.e., current I_(111-4a) if FIG. 3(E)and current I_(111-4b) in FIG. 3(F)) passing through MTJ element 111. Inaddition, according to a presently preferred embodiment, the initialreadout process is conducted after MTJ element has cooled substantiallyto room temperature (indicated in FIGS. 3(E) and 3(F) by “T₁₁₁₋₄=RT”)before measuring read current I_(111-4x). As indicated in FIG. 3(E),when the final resistance state R_(111-4a) has not switched (e.g., whenthe resistance value of final resistance state R_(111-4a) remainslow/R_(MIN) because storage magnetic orientation M_(130-4a) remainsparallel to reference magnetic orientation M₁₂₀), read currentI_(111-4a) has a relatively high current level. Conversely, as indicatedin FIG. 3(F), when the final resistance state R_(111-4b) switched (e.g.,when the resistance value of final resistance state R_(111-4a) ishigh/R_(MAX) because storage magnetic orientation M_(130-4b) switches toan anti-parallel direction relative to reference magnetic orientationM₁₂₀), corresponding read current I_(1111-4b) has a relatively lowcurrent level. Thus, the final resistance values R_(111-4a) andR_(111-4b) in each case can be determined by way of determining thehigh/low current level of corresponding read currents I_(111-4a) andI_(111-4b).

Referring to the bottom of FIG. 2, in one embodiment on-chip circuitry(e.g., gas level processing sub-circuit 173, see FIG. 1) is utilized togenerate measured gas concentration level data in accordance with theswitched/non-switched final resistive state R₁₁₁ determined during thereadout phase. As indicated in FIG. 1, in one specific embodiment, gaslevel processing sub-circuit 173 configured to receive read currentmeasurement data from MTJ element measurement sub-circuit 171, todetermine the high/low resistance state of resistance state R₁₁₁ usingthe read current measurement data, and then to generate measured gasconcentration level data is generated in a form that can be transmitted(output) to and utilized by an external system. In another embodiment,the final resistance value may be determined by way of comparing thefinal read current (i.e., currents I_(111-4a) or I_(111-4b), shown inFIGS. 3(E) and 3(F), respectively) with the initial read current I₁₁₁₋₂(FIG. 3(B)) generated at the end of the reset phase to determine whetherthe final resistance state of MTJ element 111 had switched during thegas sensing phase.

FIG. 4 is a simplified cross-sectional view showing a gas sensor 100Aaccording to an exemplary practical embodiment of the present invention.Gas sensor 100A is functionally essentially identical to gas sensor 100of FIG. 1, so for brevity various simplifications are utilized tominimize repeating description provided above. For example, variouselements of gas sensor 100A that are functionally identical tocorresponding elements of gas sensor 100 are identified using the samereference number with the postscript letter A (e.g., MTJ element 111A ofgas sensor 100A is understood as being functionally and structurally thesame as MTJ element 111 of gas sensor 100), and it is understood thatdetails provided above regarding elements of gas sensor 100 apply to thecorresponding elements of gas sensor 100A. In addition, the controlcircuit of gas sensor 100A is referenced as including frontend circuitstructures 170A-1 and backend structures 170A-2 for reasons explainedbelow, but it is understood that the control circuit of gas sensor 100Aincludes the same sub-circuits discussed above with reference to gassensor 100.

Referring to FIG. 4, sensor 100A differs from gas sensor 100 in that gassensing element 150A and MTJ elements 111A are fabricated on a membranestructure 135A utilizing a modified CMOS fabrication flow that achievessuperior performance by providing thermal isolation between the highreaction temperatures occurring at gas sensing element 150A and thecurrent measurement transistors forming the control circuit of gassensor 100A. Specifically, frontend structures 170A-1 of the controlcircuit (e.g., NMOS and CMOS transistors) are fabricated on a bulkmonocrystalline silicon substrate 301A using substantially standard CMOSfrontend processing techniques in a region (lateral area) of the CMOS ICstructure forming gas sensor 100A that is identified in FIG. 4 as “CMOSAREA”, and then backend structures 170A-2 of the control circuit (e.g.,metallization lines, vias and contacts) are fabricated in a back endstack 310A formed on the silicon substrate 301A over the frontendstructures 170A-1, also in accordance with standard CMOS fabricationtechniques. The standard CMOS backend process flow and, in some cases,the latter part of the standard CMOS frontend process flow, is/aremodified to facilitate fabricating MTJ element 111A, gas sensing element150A, field line structure 140A, heating element 145A, and any othersensor elements (e.g., temperature sensors) that function to generatethe thermal-reaction-type gas sensing operation in separate lateral areaof the CMOS IC structure identified in FIG. 4 as “GAS SENSING AREA” thatis disposed next to region CMOS AREA. In one embodiment, heating elementstructure 145A is fabricated on FOX or STI 304A on an upper surface 302Aof bulk silicon substrate 301A using the same materials that are used infront end processing (e.g. polycrystalline silicon, Titanium (Ti),Tungsten (W), silicoses, etc.) that are used during frontend processing,then (third) metal lines 115A-31 and 115A-32 are formed, e.g., usingmetallization layer M1, that extend between regions GAS SENSING AREA andCMOS AREA (i.e., onto membrane structure 305A) to provide signal linesfor transmitting heater current I_(145A) between the CMOS controlcircuit (e.g., frontend structures 170A-1) and heating element 145A.Field line structure 140A is formed in back end stack 310A over heatingelement 145A using one of the metallization layers (e.g., M1 or M2)between passivation layers, and (second) metal lines 115A-21 and 115A-22are formed, e.g., using metallization layers M1 to M3, that extendbetween regions GAS SENSING AREA and CMOS AREA (i.e., onto membranestructure 305A) to provide signal lines for transmitting field linecurrent I_(140A) between frontend structures 170A-1 and field linestructure 140A. MTJ element 111A is then formed, along with (first)metal lines 115A-11 and 115A-12 that extend across membrane structure315A to provide signal lines for transmitting read current I_(111A)between frontend structures 170A-1 and MTJ 111A. Finally, gas sensingelement 150A is formed on an upper surface 311A of back end stack layer310A, over MTJ element 111A. After backside processing is completed, acavity 305A is formed below lateral area GAS SENSING AREA by way of wetor dry etching through a lower substrate surface 303A of bulk siliconsubstrate 301A, with the etching controlled to stop at FOX or STI 304Adisposed below heating element 145A, whereby the portion of back endstack 310A located over cavity 305A defines thermally isolated membranestructure 315A. In the embodiment shown in FIG. 4, membrane structure305A is formed as a closed-type membrane, which minimizes the number ofprocess steps needed to form the membrane structure. In an alternativeembodiment (not shown), a suspended membrane structure is formed usingan additional mask that is required to form holding arms that suspendthe membrane structure. In either case, membrane structure 315A isformed with a small thermal mass that facilitates rapidly setting thetemperature of sensor 100A, and also serves to avoid heating controlcircuitry 170A-1 during sensor operation. Moreover, because gas sensor100A is fabricated using only a few changes to otherwise standard CMOSfabrication process flow, gas sensor 100A is produced at substantiallylower cost than conventional SOI-based gas sensors. That is, althoughthe fabrication of MTJ sensing elements during back end processing of acore CMOS process flow requires the addition of three masks to the coreCMOS process flow, this modification is significantly less than the costof using SOI substrates as a starting material, and thus allowsfabricating gas sensors that not only have performance advantages overconventional devices, but are also cheaper than competing solutions.

FIG. 5 is a simplified perspective view showing a semiconductor gassensor 100B including two MTJ elements 111B-1 and 111B-2 that areconfigured in the manner described above with reference to MTJ element111 (FIG. 1) and are operably coupled to a gas sensing element 150A suchthat, similar to the first embodiment mentioned above, each MTJ element111B-1 and 111B-2 switches its associated resistance state (R_(111B-1)and R_(111B-2), respectively) in response to an ambient concentrationlevel C of a target gas G in an environment E containing gas sensor100B. For example, MTJ element 111B-1 includes a storage layer 130B-1separated from a reference layer 120B-1 by a tunnel dielectric layer114B-1, where storage layer 130B-1 and reference layer 120B-1 comprisemulti-layer structures configured to switch from an initial magneticorientation (e.g., parallel) to an opposite (e.g., anti-parallel)magnetic orientation when gas sensing element 150A is exposed to targetgas G having a concentration level above a predetermined concentrationlevel. Gas sensor 100B also includes a control circuit 170B that isconfigured to implement an operating cycle similar to that describedabove in order to determine final resistance values of resistance statesR_(111B-1) and R_(111B-2) after each gas sensing phase, for example, byway of controlling a field line structure 140B and a heating element145B that are constructed and function in a manner similar to thatdescribed above with reference to gas sensor 100.

According to a novel feature of gas sensor 100B, gas sensor 100B isconfigured such that, during the gas sensing phase of the sensoroperating cycle, resistance state R_(111B-1) of MTJ element 111B-1switches at a relatively low (first) corresponding target gasconcentration level, and resistance state R_(111B-2) of MTJ element111B-2 switches at a relatively high (second) target gas concentrationlevel—that is, MTJ element 111B-1 switches at a corresponding gasconcentration level that is different from that of MTJ element 111B-2.By configuring gas sensor 100B such that MTJ elements 111B-1 and 111B-2switch at different corresponding gas concentration levels, gas sensor100B is configured to quantitatively determine an actual (i.e.,currently measured) gas concentration level to within gas concentrationlevel range defined by the respective switching concentration levels atwhich MTJ element 111B-1 and 111B-2 switch their resistance states. Thatis, as explained below with reference to FIGS. 7(A) to 7(C), identifyingthe switched/not-switched status of each MTJ element 111B-1 and 111B-2after a gas sensing phase indicates whether the actual gas concentrationlevel is below the relatively low target gas concentration level atwhich MTJ element 111B-1 switches its resistance state, between therelatively low and relatively high target gas concentration levels atwhich MTJ elements 111B-1 and 111B-2 respectively switch resistancestates, or above the relatively high target gas concentration level.This concept is also scalable to provide highly accurate gasconcentration level measurements by way of increasing the number of MTJelements that switch resistance states at respective different gasconcentration levels, and setting the respective different gasconcentration levels such that the gas concentration range between eachadjacent pair of respective different gas concentration levels isvanishingly small (i.e., such that the difference between the storageblocking temperatures of the MTJ elements is very small).

According to a novel feature of gas sensor 100B, field line structure140B is operably magnetically coupled to both MTJ elements 111B-1 and111B-2 such that, when actuated, field line structure 140B generates amagnetic field F having a sufficiently strong magnetic bias force tosimultaneously bias the storage magnetic orientations of the storagelayers of both MTJ elements 111B-1 and 111B-2 in accordance with amagnetic direction of magnetic field F. In one embodiment, shared fieldline structure 140B consists of a single integral metal structure thatextends linearly under the MTJ elements 111B-1 and 111B-2, and iscontrolled by way of an applied read line current I₁₄₀ generated byfield line control sub-circuit 175B of control circuit 100B. Because asingle magnetic field F simultaneously controls both MTJ elements 111B-1and 111B-2, field line structure 140B is effectively “shared” by MTJelements 111B-1 and 111B-2, and therefore differs from other MTJarrangements (e.g., magnetic logic unit (MLU) devices that requireseparately controlled field lines for each MTJ element). Note that the“shared” field line arrangement utilized by gas sensor 100B isexpandable to any number of MTJ elements by aligning the MTJ elementslinearly and/or by configuring “shared” field line structure 140B toapply a common magnetic bias force onto all of the MTJ elements. Byutilizing shared field line structure 140B to control MTJ elements111B-1 and 111B-2 (or a larger number of MTJ elements), only a singlefield line control signal is required to control multiple MTJ elementsduring the gas sensor's operating cycle, thereby reducing controlcircuit complexity. Further, because any number of MTJ elements can becontrolled using a single shared field line, gas sensors of the presentinvention implementing this shared field line approach are scalable toinclude any number of MTJ elements without requiring additional signallines or other modifications to the control circuitry, therebyfacilitating scalable quantitative gas concentration measurementresolution without increasing operating complexity. Of course, multiplefield lines could be used in place of shared field line 140B, eachcontrolling one MTJ element.

Note that gas sensor 100B differs from gas sensor 100 (FIG. 1) in thatgas sensing element 150B is not necessarily restricted to athermal-reaction-type gas sensing element, and may be implemented usingone or more a chemical-reaction-type gas sensing elements. In each case,gas sensing element 150B is operably coupled to MTJ elements 111B-1 and111B-2, and functions to effect resistance state switching in responseto corresponding gas concentration levels. In the case where gas sensingelement 150B is implemented using one or more thermal-reaction-type gassensing element, gas sensing element 150B is operably thermally coupledto MTJ elements 111B-1 and 111B-2, and reaction heat generated by gassensing element 150B is used to cause switching of resistance statesR_(111B-1) and R_(111B-2) of MTJ elements 111B-1 and 111B-2,respectively, in a manner similar to that described above with referenceto gas sensor 100. In the case where gas sensing element 150B isimplemented using one or more chemical-reaction-type gas sensingelements, gas sensing element 150B is operably physically coupled to MTJelements 111B-1 and 111B-2 such that changes in the composition orchemical structure of gas sensing element 150B in response to adsorptionof target gas G causes switching of resistance states R_(111B-1) andR_(111B-2) of MTJ elements 111B-1 and 111B-2, respectively.

According to another novel feature of gas sensor 100B, MTJ elements111B-1 and 100B-2 are disposed in a series-connected NAND-type string110B, which in the exemplary embodiment is connected between voltagesource V₁₁₀ and ground by way of a select transistor 117B such that aread current I₁₄₀ passes sequentially through MTJ elements 111B-1 and100B-2. With this arrangement, series-connected string 110B exhibits atotal string resistance R_(110B) that is collectively defined by (i.e.,in this example, a sum of) corresponding respective resistance valuesR_(111B-1) and R_(111B-2) of MTJ elements 111B-1 and 111B-2. In a mannersimilar to that described above, control circuit 170B includes a stringcontrol sub-circuit 171B that functions to generate and measure readcurrent I₁₄₀ during reset and readout operations, thereby providing bothan initial string resistance value R_(111BI) and a final stringresistance value R_(111BI) that can be used to determine the individualresistance states R_(111B-1) and R_(111B-2) of MTJ elements 111B-1 and111B-2. In the illustrated embodiment, control circuit 170B utilizes agas sensing processing sub-circuit 179B including memory devices MD1 andMD2 for respectively storing resistance values R_(111BI) and R_(111BI),a processor P configured to calculate a difference R_(DIFF) between theinitial and final resistance values, and a look-up table LUT configuredto store predetermined gas concentration level data values SGCLV. Withthis arrangement, calculated difference value R_(DIFF) (e.g., 0, 1 or 2)is utilized to access a corresponding gas concentration level valuestored in lookup table LUT (i.e., less than lower value C1, betweenvalues C1 and C2, or above value C2), and the corresponding gasconcentration level value is then transmitted as the output measured gasconcentration level data via an output port OP to an external system(not shown).

FIG. 6 shows a thermal-reaction-type semiconductor gas sensor 100Cincluding two MTJ elements 111C-1 and 111C-2 that are operably thermallycoupled to a thermal-reaction-type gas sensing element 150A andconnected in a series-connected string 110C. Similar to gas sensor 100B,each MTJ element 111C-1 and 111C-2 is consistent with MTJ element 111 ofFIG. 1, and switches its associated resistance state R_(111C-1) andR_(111C-2) in response to different ambient target gas concentrationlevel. In addition, gas sensor 100C includes a shared field linestructure 140C and a shared heating element 145C that function in themanner described above.

According to a feature of gas sensor 100C, MTJ elements 111C-1 and111C-2 are maintained at substantially identical temperatures during thesensor operating cycle, and are configured to switch at different gasconcentration levels by way of having respective storage layers 130C-1and 130C-2 that exhibit different storage blocking temperaturesT_(B130C-1) and T_(B130C-2) respectively (e.g., wherein storage blockingtemperature TB_(130C-1) is 25° C. lower than storage blockingtemperature T_(B130C-2)). That is, shared heating element 145C and gassensing element 150A are respectively thermally coupled and otherwiseconfigured to respectively generate control and reaction heat in amanner similar to that described above, but in this case the heat isgenerated such that MTJ temperature T_(111C-1) and T_(111C-2) of MTJelements 111C-1 and 111C-2, respectively, are maintained at essentiallyidentical temperature levels during the reset and gas sensing phases.Providing MTJ elements 111C-1 and 111C-2 with different storage blockingtemperatures T_(B130C-1) and T_(B130C-2) can be achieved by way ofintentional and/or non-intentional (inherent) mechanisms. For example,in the exemplary embodiment shown in FIG. 6, MTJ elements 111C-1 and111C-2 are intentionally provided with different storage blockingtemperatures T_(B130C-1) and T_(B130C-2) by way of controlling theapplied fabrication process used to produce gas sensor 100C such thateach MTJ element 111C-1 and 111C-2 respectively has a different lateralsize W_(111C-1) and W_(111C-2). In one embodiment, lateral sizesW_(111C-1) and W_(111C-2) vary in the range of 100 nm and 500 nm. Inother embodiments, different storage blocking temperatures areintentionally achieved by forming MTJ elements having differentanti-ferromagnetic (AFM) layer thickness. In yet other embodiments, theMTJ elements are generated using the same processing parameters (e.g.,same lateral width and thickness), and inherent blocking temperaturedistributions, which typically vary in the range of 50-100° C. due tofluctuations of the grain sizes in the AFM layers forming the storageand/or reference layers, are utilized to provide the desired differentstorage blocking temperatures.

An exemplary operation cycle of gas sensor 100C is depicted withreference to FIGS. 6 and 7(A) to 7(C), where FIG. 6 shows gas sensor100C during a reset phase (i.e., at a time t1), and FIGS. 7(A) to 7(C)depict gas sensor 100C during alternative gas reaction phases (i.e., ata time t2 after time t1) in which gas sensor 100C is exposed to threedifferent target gas concentration levels CA0, CA1 and CA2, respectively(i.e., FIG. 7(A) depicts gas sensor 100C exposed to target gasconcentration CA0 at associated time t2 a, FIG. 7(B) depicts gas sensor100C exposed to target gas concentration CA1 at associated time t2 b,and FIG. 7(C) depicts gas sensor 100C exposed to target gasconcentration CA2 at associated time t2 c).

Referring to FIG. 6, during the reset phase a first amount of controlheat H₁₄₅₋₁ is generated (e.g., by actuating heating element 145C usinga first heating current I₁₄₅₋₁) such that MTJ temperatures T_(111C-1)and T_(111C-2) of MTJ elements 111C-1 and 111C-2 increase above bothassociated storage blocking temperatures T_(B130C-1) and T_(B130C-2),and then field line structure 140C is activated (e.g., by way of a firstfield line current I₁₄₀₋₁) to generate a magnetic field F1, wherebystorage magnetic orientations M_(130C-1) and M_(130C-2) of storagelayers 130C-1 and 130C-2, respectively, are biased into parallel (first)directions relative to reference magnetic orientations M₁₂₀ of referencelayers 120C-1 and 120C-2. At the end of the reset phase, MTJ elements111C-1 and 111C-2 are cooled below both associated storage blockingtemperatures T_(B130C-1) and T_(B130C-2) (preferably to roomtemperature), whereby storage magnetic orientations M_(130C-1) andM_(130C-2) are fixed in the parallel direction, and an initialresistance value R_(110CI) of series-connected string 110C isdetermined, e.g., by way of measuring a read current I_(110CI) that isgenerated using methods described above (note that read currentI_(110CI) is generated after the reset operation, but is indicated inFIG. 6 for reference). In one embodiment, initial resistance valueR_(110CI) is stored in an on-chip memory location (e.g., memory deviceMD1, shown in FIG. 5). In this case, because both MTJ elements 111C-1and 111C-2 are fixed in parallel directions, initial resistance valueR_(110CI) has a minimum resistance value R_(MIN).

FIGS. 7(A) to 7(C) depict gas sensor 100C exposed to target gasconcentration levels CA0, CA1 and CA2, respectively, during alternativegas reaction phases. Referring to FIG. 7(A), the gas sensing phaseinvolves heating MTJ elements 111C-1 and 111C-2 to a work pointtemperature (and subsequently maintaining MTJ elements 111C-1 and 111C-2at the work point temperature) while applying a magnetic bias force suchthat at least one of storage magnetic orientations M_(130C-1) andM_(130C-2) switches from the parallel (first) direction to ananti-parallel (second) direction when target gas G is present inenvironment E in an amount above a predetermined minimum gasconcentration level. Increasing/maintaining the temperature of MTJelements 111C-1 and 111C-2 to/at the work point temperature involvesre-activating heating element 145C by way of an associated heatercurrent I₁₄₅₋₂ such that control heat H₁₄₅₋₂ maintains MTJ temperaturesT_(111C-1) and T_(111C-2) at the work point temperature, which isdescribed in additional detail below with reference to FIG. 8. Applyingthe opposite magnetic bias force involves operating gas sensor 100C suchthat storage magnetic orientations M_(130C-1/2) are biased in theanti-parallel direction relative to reference magnetic orientations M₁₂₀by a magnetic bias force directed opposite to the magnetic bias forceapplied during the reset phase. In one embodiment, the magnetic biasforce utilized during the gas sensing phase is generated usinganti-parallel magnetization force AMV, spin torque transfer STT and/orby generating a (second) external field F2, where using magnetic forceAMV requires configuring MTJ elements 111C-1 and 111C-2 in the mannerdescribed above, and generating a (second) external field F2 involves,for example, passing a field line current I₁₄₀₋₂ along field line 140Cin a direction opposite to field line current I₁₄₀₋₁ during the resetphase (see FIG. 6). Under these conditions, as explained below withreference to the examples shown in FIGS. 7(A) to 7(C), at least one ofstorage magnetic orientations M_(130C-1) and M_(130C-2) is caused toswitch from the initial parallel direction to the anti-parallel(opposite) direction when target gas G is above the predeterminedminimum gas concentration level corresponding to storage blockingtemperature T_(B130C-1) of MTJ element 111C-1.

FIG. 7(A) shows gas sensor 100(t 2 a) during a first exemplary gassensing phase in which gas sensing element 150C is exposed to anenvironment E1 containing target gas G in a relatively low (or zero)concentration level CA0. As mentioned above, MTJ elements 111C-1 and111C-2 are increased/maintained at the work point temperature by way ofcontrol heat H₁₄₅₋₂ generated by heating element 145C. Target gasconcentration level CA0 causes gas sensing element 150C to generatereaction heat H_(150-2a) that either fails to increase MTJ temperaturesT_(111C-1) and T_(111C-1) above the work point temperature, or raisesMTJ temperatures T_(111C-1) and T_(111C-1) by an amount that remainsbelow storage blocking temperature T_(B130C-1) of MTJ element 111C-1.Because neither MTJ element reaches its storage blockage temperature,storage magnetic orientations M_(130C-1) and M_(130C-2) remain fixed inthe parallel direction relative to reference magnetic orientationM_(120C) (i.e., resistance states R_(111C-1) and R_(111C-2) retain lowresistance values).

FIG. 7(B) shows gas sensor 100(t 2 b) during an alternative (second)exemplary gas sensing phase in which gas sensing element 150C is exposedto an environment E2 containing target gas G in a concentration levelCA1 corresponding to the predetermined minimum gas concentration levelof MTJ element 111C-1. As in the first example, MTJ elements 111C-1 and111C-2 are increased/maintained at the work point temperature by way ofcontrol heat H₁₄₅₋₂ generated by heating element 145C. In this case,target gas concentration level CA1 causes gas sensing element 150C togenerate reaction heat H_(150-2b) in an amount that raises MTJtemperatures T_(111C-1) and T_(111C-1) to storage blocking temperatureT_(B130C-1) of MTJ element 111C-1, whereby the applied magnetic biasforce AMV/STT/F2 causes storage magnetic orientation M_(130C-1) toswitch from the parallel direction to the anti-parallel directionrelative to reference magnetic orientation M_(120C), whereby resistancestate R_(111C-1) of MTJ element 111C-1 changes from its initial lowresistance value to a high resistance value. Because MTJ element 111C-2remains below storage blocking temperature T_(B130C-2), resistance stateR_(111C-2) retains its initial low resistance value.

FIG. 7(C) shows gas sensor 100(t 2 c) during an alternative (third)exemplary gas sensing phase in which gas sensing element 150C is exposedto an environment E3 containing target gas G in a relatively highconcentration level CA2 corresponding to the predetermined minimum gasconcentration level of MTJ element 111C-2. As in the previous examples,MTJ elements 111C-1 and 111C-2 are increased/maintained at the workpoint temperature by way of control heat H₁₄₅₋₂ generated by heatingelement 145C. Target gas concentration level CA2 causes gas sensingelement 150C to generate reaction heat H_(150-2c) in an amount thatraises MTJ temperatures T_(111C-1) and T_(111C-1) to storage blockingtemperature T_(B130C-2) of MTJ element 111C-2. Because MTJ elements111C-1 and 111C-2 both reach temperatures above their respective storageblocking temperatures, both storage magnetic orientations M_(130C-1) andM_(130C-2) switch from parallel to anti-parallel directions relative toreference magnetic orientation M_(120C), whereby resistance statesR_(111C-1) and R_(111C-2) of MTJ elements 111C-1 and 111C-2 change fromlow to high resistance values.

After each of the alternative gas sensing phases described above withreference to FIGS. 7(A) to 7(C), MTJ elements 111C-1 and 111C-2 arecooled substantially to room temperature (e.g., until MTJ temperaturesT_(111C-1) and T_(111C-2) are approximately 25° C.), and a readout phaseis then performed to determine the final string resistance value and togenerate/output corresponding measured gas concentration level data.Similar to the readout processes described above, reading final stringresistance is achieved by applying a fixed read voltage and measuringthe resulting read current. As indicated in FIG. 7(A), because neitherMTJ element 111C-1 and 111C-2 switched resistance states during the gassensing phase in response to gas concentration level CA0, read currentI_(110CFa) generated during the readout phase is substantially at thesame high current level as that of read current I_(110CI), which wasgenerated at the end of reset (see FIG. 6), so final string resistancevalue R_(110CFa) is also equal to minimum resistance value R_(MIN). Incontrast, referring to FIG. 7(B), because MTJ element 111C-1 switchedits resistance state during the gas sensing phase in response to gasconcentration level CA1, read current I_(110CFb) has a level that islower than read current I_(110CI) by an amount corresponding to theincreased resistance state of one MTJ element, final string resistancevalue R_(110CFb) is assigned an intermediate value R_(INT). Referring toFIG. 7(C), because both MTJ elements switched resistance states inresponse to gas concentration level CA2, read current I_(110CFc) has alevel corresponding to the increased resistance state of two MTJelements, final string resistance value R_(110CFc) is assigned a maximumresistance value R_(MAX). Generating and outputting measured gasconcentration level data is then generated, for example, using theprocess described above with reference to gas level processingsub-circuit 179B (FIG. 5).

FIG. 8 is simplified graph that relates MTJ temperatures T_(111C-1) andT_(111C-2) and total string resistance R_(110C) during the exemplary gassensing phase of gas sensor 100C described above with reference to FIGS.7(A) to 7(C), and in particular illustrates exemplary work pointtemperature T_(WP) (i.e., zero target gas concentration point) inrelation to storage blocking temperatures T_(B130C-1) and T_(B130C-2),and the corresponding measurement range of gas sensor 100C. As discussedabove, work point temperature T_(WP) is generally defined as the lowesttemperature at which gas sensing element 150C (e.g., FIG. 7(A))generates reaction heat in response to a selected target gas (i.e., thezero gas concentration level). That is, if gas sensor 100C is attemperature below the work point temperature, gas sensing element 150Cwould not react to the presence of the selected target gas. Also, forsensor 100C to function properly, MTJ elements 111C-1 and 111C-2 (e.g.,FIG. 7(A)) must have corresponding storage blocking temperaturesT_(B130C-1) and T_(B130C-2) that are above work point temperatureT_(WP). The temperature differences required to change the total stringresistance of string 110C are indicated in FIG. 8 as ΔT1 and ΔT2, wheretemperature difference ΔT1 is the difference between work pointtemperature T_(WP) and storage blocking temperature T_(B130C-1), and istherefore relatively small in comparison to temperature difference ΔT2between work point temperature T_(WP) and storage blocking temperatureT_(B130C-2). Applying the graphic description of FIG. 8 to the exemplarygas sensing phase of FIG. 7(A), when gas sensor 100C is exposed totarget gas G below concentration level CA1 (i.e., level CA0), reactionheat H_(150C-2a) generated by gas sensing element 150C increases MTJtemperatures T_(111C-1) and T_(111C-2) by an amount less than ΔT1 (i.e.,by an amount insufficient to increase from the work point temperature tostorage blocking temperature T_(B130C-1)), whereby neither MTJ element111B-1 switches its resistance state, leaving string resistanceR_(111CFa) at R_(MIN). Referring to FIG. 7(B), when gas sensor 100C isexposed to target gas G at concentration level CA1, reaction heatH_(150C-2b) generated by gas sensing element 150C is sufficient toincrease MTJ temperatures T_(111C-1) and T_(111C-2) by amount ΔT1 fromthe work point temperature to storage blocking temperature T_(B130C-1),whereby only MTJ element 111B-1 switches its resistance state, changingstring resistance R_(111CFb) to R_(MIN). Referring to FIG. 7(C), whengas sensor 100C is exposed to target gas G at concentration level CA2,reaction heat H_(150C-2C) is sufficient to increase MTJ temperaturesT_(111C-1) and T_(111C-2) by amount ΔT2 from the work point temperatureto storage blocking temperature T_(B130C-2), whereby both MTJ elements111B-1 and 111B-2 switches resistance states, changing string resistanceR_(111CFc) to R_(MAX).

FIG. 9 depicts a gas sensor 100D including MTJ elements disposed inthree series-connected strings 110D-1, 110D-2 and 110D-3 that are alsoconnected in parallel between a voltage source V₁₁₀ and a selecttransistor 117D. Each string 110D-1 includes multiple MTJ elements thatare configured to switch at different gas concentration levels (e.g.,string 110D-1 includes MTJ elements 111D-11 to 111D-18, string 110D-2includes MTJ elements 111D-21 to 111D-28, and string 110D-3 includes MTJelements 111D-31 to 111D-38), where each of the MTJ elements isconfigured as described above. Gas sensor 100D also includes a gassensing element 150D, a field line 140D and a heating element 145D thatare depicted in simplified form for clarity. Gas sensor 100D illustratesone example of how gas sensors of the present invention are readilyscalable to provide a range of measurement accuracies by way ofincreasing the number of MTJ elements in each series-connected stringand/or connecting multiple series strings in parallel, thereby providinggas sensors capable of measuring very small gas concentration levelvariations. Note that the total string resistance of each parallelseries-connected string 111D-1 to 111D-3 is collectively defined by asum of each strings MTJ elements 111D-11 to 111D-18, 111D-21 to 111D-28and 111D-31 to 111D-38, respectively, and that a total resistance of theentire parallel-connected-string structure is also collectively definedby the MTJ elements of the three strings. The parallel-connected-stringarrangement also facilitates measuring multiple target gases, where eachtarget gas has a different combustion temperature (i.e., the temperaturecorresponding to the “0” gas concentration level of FIG. 8). In thiscase, gas sensor 100D may be implemented to measure, for example, threedifferent target gases by way of actuating heating element 145D to heatthe parallel-connected-string structure to a first predetermined gasconcentration level optimized to measure a first target gas having ameasurement range corresponding to storage blocking temperatures of MTJelements 111D-11 to 111D-18 of string 110D-1, then heating theparallel-connected-string structure to a second (higher) predeterminedgas concentration level optimized to measure a second target gas havinga measurement range corresponding to storage blocking temperatures ofMTJ elements 111D-21 to 111D-28 of string 110D-2, and then heating theparallel-connected-string structure to a third (yet higher)predetermined gas concentration level optimized to measure a thirdtarget gas having a measurement range corresponding to storage blockingtemperatures of MTJ elements 111D-31 to 111D-38 of string 110D-3.

FIG. 10 depicts a gas sensor 100E including a CMOS control circuit 170Emade up of frontend structures 170E-1 disposed on a silicon substrate301E and backend structures 170E-2 disposed in a back end stack 310Eformed on the silicon substrate 301E, and multiple MTJ elements 111E-11to 111E-34 and three gas sensing elements 150E-1 to 150E-3 respectivelydisposed in groups on membrane structures 315E-1 to 315E-3. Similar tothe embodiment described above with reference to FIG. 4, each membranestructure 315E-1 to 315E-3 comprises a respective portion of back endstack 310E and is disposed over a corresponding cavity 305E-1 to 305E-3defined in silicon substrate 301E. In the disclosed embodiment, thegroups of MTJ elements respectively disposed on membrane structures315E-1 to 315E-3 are series-connected in NAND-type strings 110E-1 to110E-3 that are respectively magnetically coupled to shared field linestructures 140E-1 to 140E-3 and thermally coupled to associated heatsensing elements 150E-1 to 150E-3 and heating elements 145E-1 to 145E-3.Specifically, membrane structure 315E-1 includes series-connected string110E-1 including MTJ elements 111E-11 to 111E-14 that are magneticallycoupled to shared field line structure 140E-1 and thermally coupled toassociated heat sensing element 150E-1 and heating element 145E-1,membrane structure 315E-2 includes string 110E-2 including MTJ elements111E-21 to 111E-24, shared field line structure 140E-2, heat sensingelement 150E-2 and heating element 145E-2, and membrane structure 315E-3includes string 110E-3 including MTJ elements 111E-31 to 111E-34, sharedfield line structure 140E-3, heat sensing element 150E-3 and heatingelement 145E-3. In addition, gas sensor 100E is configured tosimultaneously quantitatively measure three different target gases byway of configuring each group of MTJ elements to switch resistancestates in response to different storage blocking temperatures, and byconfiguring each heating element 145E-1 to 145E-3 to maintaincorresponding groups of MTJ elements at a different work pointtemperature. To optimize the MTJ elements for the different work pointtemperatures, in one embodiment each group of MTJ elements is formedwith a different lateral size. For example, as indicated by the bubblesat the top of FIG. 10, MTJ element 111E-14 is formed with a lateral sizeW_(111E-14) that is smaller than lateral size W_(111E-24) of MTJ element111E-24 in order to provide MTJ element 111E-14 with a lower storageblocking temperature T_(B130E-14) than MTJ element 111E-24, and lateralsize W_(111E-24) of MTJ element 111E-24 is smaller than W_(111E-34) ofMTJ element 111E-34 to provide MTJ element 111E-24 with a lower blockingtemperature T_(B130E-24) than storage blocking temperature T_(B130E-34)of MTJ element 111E-34. Other approaches mentioned above may also beused to generate MTJ elements having different storage blockingtemperatures.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention.

The invention claimed is:
 1. A method for measuring a gas concentrationlevel of a target gas using at least one magnetic tunnel junction (MTJ)element and a thermal-reaction-type gas sensing element, the MTJ elementincluding a reference layer defining a reference magnetic orientationand a storage layer defining a storage magnetic orientation, thethermal-reaction-type gas sensing element being configured to generatereaction heat in an amount proportional to the concentration level ofsaid target gas, said method comprising: during a reset phase,generating a first amount of control heat such that an MTJ temperatureof said MTJ element increases above a storage blocking temperature ofsaid storage layer; applying a first magnetic bias force such that saidstorage magnetic orientation is biased into in a first directionrelative to said reference magnetic orientation by said first magneticbias force; decreasing said MTJ temperature below said storage blockingtemperature such that said storage magnetic orientation is fixed in saidfirst direction relative to said reference magnetic orientation; andduring a gas sensing phase following the reset phase, applying a secondmagnetic bias force while generating a second amount of control heatthat maintains said MTJ temperature at a work point temperature suchthat, when said measured gas concentration level of said target gas isabove a predetermined minimum gas concentration level, said reactionheat generated by said gas sensing element increases said MTJtemperature from said work point temperature to said storage blockingtemperature, wherein said second magnetic bias force is generated suchthat said storage magnetic orientation is biased into in a seconddirection relative to said reference magnetic orientation by said secondmagnetic bias force, said second direction being opposite to said firstdirection such that said storage magnetic orientation switches from saidfirst direction to said second direction when said target gas is abovesaid predetermined minimum gas concentration level.
 2. The method ofclaim 1, wherein generating said first amount of control heat comprisesactivating a resistive heating element that is operably thermallycoupled to said MTJ element, and wherein decreasing said MTJ temperaturebelow said storage blocking temperature comprises de-activating saidresistive heating element.
 3. The method of claim 2, wherein generatingsaid second amount of control heat comprises re-activating saidresistive heating element.
 4. The method of claim 1, wherein applyingsaid first magnetic bias force comprises activating a field linestructure that is operably magnetically coupled to said MTJ element suchthat said field line structure generates a first magnetic field havingsufficient magnetic force to bias the storage magnetic orientation ofthe storage layer into said first direction relative to said referencemagnetic orientation.
 5. The method of claim 1, wherein applying saidsecond magnetic bias force comprises at least one of activating a fieldline structure that is operably magnetically coupled to said MTJ elementsuch that said field line structure generates a second magnetic field,and configuring said MTJ element such that said reference layertransfers one of an anti-parallel alignment of magnetization vectors(AMV) force and a spin torque transfer that acts as the second magneticbiasing force.
 6. The method of claim 1, further comprising: during areadout phase following the gas sensing phase, decreasing said MTJtemperature below said storage blocking temperature, and determining afinal resistive state of said MTJ element by measuring a first readcurrent passing through said MTJ element; and generating measured gasconcentration level data in accordance with said determined finalresistive state.
 7. The method of claim 6, further comprising, afterdecreasing said MTJ temperature during said reset phase, determining aninitial resistive state of said MTJ element by measuring a second readcurrent passing through said MTJ element, wherein generating saidmeasured gas concentration level data comprises utilizing both saidfirst read current and said second read current.
 8. The method of claim7, wherein decreasing said MTJ temperature during said reset phase andduring said readout phase comprises decreasing said MTJ temperaturesubstantially to room temperature before measuring said second and firstread currents, respectively.
 9. A method for measuring a gasconcentration level of a target gas using a gas sensor includingplurality of magnetic tunnel junction (MTJ) element and at least onethermal-reaction-type gas sensing element, each said MTJ elementincluding a reference layer defining a reference magnetic orientationand a storage layer defining a storage magnetic orientation, saidthermal-reaction-type gas sensing element being configured to generatereaction heat in an amount proportional to the concentration level ofsaid target gas, said method comprising: during a reset phase,generating a first amount of control heat such that an MTJ temperatureof each of said plurality of MTJ elements temporarily increases above anassociated storage blocking temperature of said storage layer of saideach of said plurality of MTJ elements, and activating a single fieldline structure that is operably magnetically coupled to all of saidplurality of MTJ elements such that said field line structure generatesa first magnetic field having sufficient magnetic force to bias thestorage magnetic orientations of the storage layer of all of saidplurality of MTJ elements into said first direction relative to saidreference magnetic orientation; during a gas sensing phase following thereset phase, applying a magnetic bias force to each of said plurality ofMTJ elements while maintaining said MTJ temperatures at a work pointtemperature and below said storage blocking temperatures such that whensaid measured gas concentration level of said target gas is above apredetermined minimum gas concentration level, said reaction heatgenerated by said gas sensing element increases said MTJ temperatures ofsaid plurality of MTJ elements from said work point temperature abovethe associated storage blocking temperature of at least one of saidplurality of MTJ elements, wherein applying said magnetic bias forcecomprises operating said gas sensor such that said storage magneticorientations are biased into a second direction relative to saidreference magnetic orientations by said magnetic bias force, said seconddirection being opposite to said first direction such that said storagemagnetic orientations of said at least one of said plurality of MTJelements switches from said first direction to said second directionwhen said target gas is above said predetermined minimum gasconcentration level.
 10. The method of claim 9, wherein generating saidfirst amount of control heat comprises activating a resistive heatingelement that is operably thermally coupled to said plurality of MTJelements.
 11. The method of claim 10, wherein generating said secondamount of control heat comprises re-activating said resistive heatingelement.
 12. The method of claim 9, wherein generating said magneticbias force comprises configuring each of said plurality of MTJ elementssuch that said reference layer of said each MTJ element generates one ofan anti-parallel alignment of magnetization vectors (AMV) force or aspin torque transfer configured to bias the storage magnetic orientationof the storage layer of said each MTJ element into said second directionrelative to said reference magnetic orientation.
 13. The method of claim9, wherein generating said magnetic bias force comprises activating saidsingle field line structure such that said field line structuregenerates a second magnetic field having sufficient magnetic force tobias the storage magnetic orientations of the storage layer of each ofsaid plurality of MTJ elements into said second direction relative tosaid reference magnetic orientation.
 14. The method of claim 9, whereinsaid plurality of MTJ elements are disposed in a series-connected stringsuch that a total string resistance of said series-connected string iscollectively defined by the corresponding resistance values of saidplurality of MTJ elements, and wherein said method further comprises,during a readout phase following the gas sensing phase, decreasing saidMTJ temperatures of said plurality of MTJ elements to room temperature,and then determining a final resistance value of said series-connectedstring by measuring a first read current passing through saidseries-connected string.
 15. The method of claim 14, further comprising,before said gas sensing phase, decreasing said MTJ temperatures of saidplurality of MTJ elements to room temperature, and determining aninitial resistance value of said series-connected string by generating asecond read current passing through said series-connected string.
 16. Amethod for measuring a gas concentration level of a target gas using agas sensor including plurality of magnetic tunnel junction (MTJ) elementand at least one gas sensing element, each said MTJ element including areference layer defining a reference magnetic orientation and a storagelayer defining an associated storage magnetic orientation, wherein saidstorage layer of each said MTJ element is configured such that each saidassociated storage magnetic orientation is independently switchablebetween a parallel direction and an anti-parallel direction relative tosaid reference magnetic orientation, whereby a corresponding resistancestate of said each MTJ element is respectively switchable between afirst resistance value and a second resistance value, wherein saidplurality of MTJ elements are disposed in a series-connected string suchthat a total string resistance of said series-connected string iscollectively defined by the corresponding resistance values of saidplurality of MTJ elements, and wherein said gas sensing element isoperably coupled to said plurality of MTJ elements, said methodcomprising: during a reset phase, fixing the associated storage magneticorientations of the storage layers of all of said plurality of MTJelements into said parallel direction relative to said referencemagnetic orientation, and then determining an initial resistance valueof said series-connected string by measuring a first read currentpassing through said series-connected string; during a gas sensing phasefollowing the reset phase, operating said gas sensor such that said gassensing element causes a corresponding resistance state of each said MTJelement to switch from one of said first and second resistance values tothe other of said first and second resistance values only when saidconcentration of said target gas in said environment is at least equalto a corresponding concentration level; during a readout phase followingthe gas sensing phase, determining a final resistance value of saidseries-connected string by generating a second read current passingthrough said series-connected string; and after said readout phase,generating measured gas concentration level data in accordance with adifference between said initial resistance value and said finalresistance value.
 17. The method of claim 16, wherein, during said resetphase, determining said initial resistance value further comprisesmeasuring said first read current after said plurality of MTJ elementshave cooled substantially to room temperature, and wherein, during saidreadout phase, determining said final resistance value further comprisesmeasuring said second read current after said plurality of MTJ elementshave cooled substantially to room temperature.
 18. The method of claim17, wherein, during said reset phase, fixing the storage magneticorientations of the storage layers of all of said plurality of MTJelements into said parallel direction relative to said referencemagnetic orientation comprises activating a resistive heating elementthat is operably thermally coupled to said MTJ element, and thende-activating said resistive heating element until said MTJ elementshave cooled substantially to room temperature.
 19. The method of claim16, wherein, during said reset phase, fixing the storage magneticorientations of the storage layers of all of said plurality of MTJelements into said parallel direction relative to said referencemagnetic orientation comprises activating a single field line structuresuch that said field line structure generates a magnetic field havingsufficient magnetic force to bias the storage magnetic orientations ofthe storage layer of each of said plurality of MTJ elements into saidparallel direction relative to said reference magnetic orientation. 20.The method of claim 15, wherein generating measured gas concentrationlevel data comprises utilizing said difference between said initialresistance value and said final resistance value to access acorresponding gas concentration level data stored on said gas sensor,and then transmitting said corresponding gas concentration level datathrough an output port to an external system.